Showing papers by "Oliver Ambacher published in 2006"
TL;DR: In this article, the impact of P3HT-crystallinity on optical absorption of poly[3hexylthiophene-2,5-diyl]/[6,6]-phenyl C 61 butyric acid methyl ester-composites was investigated.
216 citations
TL;DR: In this paper, the piezoelectric coefficient d 33eff of aluminium nitride thin films was measured using both, piezoresponse force microscopy and an interferometric technique.
Abstract: The piezoelectric coefficient d 33eff of aluminium nitride thin films was measured using both, the piezoresponse force microscopy and an interferometric technique. Wurtzite AlN thin films were prepared on Si (1 1 1) substrates by reactive dc-sputtering and by metal organic chemical vapor deposition (MOCVD). Direct measurements of the inverse piezoelectric effect in the picometer range showed that the acceptable tolerance in the crystal orientation is much larger for MEMS applications than expected previously. The value of the effective piezoelectric coefficient d 33 for the prepared AlN thin films remained as high as 5.1 pm/V even for lower degrees of texture.
176 citations
TL;DR: In this article, phase selective growth of rhombohedral and cubic indium oxide polytypes was studied by adjusting substrate temperature and trimethylindium flow rate during metal organic chemical vapor deposition on c-plane sapphire.
Abstract: Phase selective growth of rhombohedral and cubic indium oxide polytypes was studied. The selective growth of different polytypes was achieved by adjusting substrate temperature and trimethylindium flow rate during metal organic chemical vapor deposition on c-plane sapphire. The optical band gaps of the cubic and rhombohedral phases were determined to be ∼3.7 and ∼3.0eV, respectively. On the basis of the performed structural investigations, a phenomenological model of the nucleation and growth of highly textured cubic In2O3 on Al2O3 (0001) is proposed.
86 citations
TL;DR: In this article, the influence of dislocations on electron transport properties of undoped InN thin films grown by molecular-beam epitaxy on AlN(0001) pseudosubstrates is reported.
Abstract: The influence of dislocations on electron transport properties of undoped InN thin films grown by molecular-beam epitaxy on AlN(0001) pseudosubstrates is reported. The microstructure and the electron transport in InN(0001) films of varying thickness were analyzed by transmission electron microscopy and variable temperature Hall-effect measurements. It was found that crystal defects have strong effects on the electron concentration and mobility of the carriers in the films. In particular, the combined analysis of microscopy and Hall data showed a direct dependence between free carrier and dislocation densities in InN. It was demonstrated that threading dislocations are active suppliers of the electrons and an exponential decay of their density with the thickness implies the corresponding decay in the carrier density. The analysis of the electron transport yields also a temperature-independent carrier concentration, which indicates degenerate donor levels in the narrow band-gap InN material. The relative in...
80 citations
TL;DR: In this article, the performance of Schottky diodes with different thickness of the catalytic metal was investigated as hydrogen gas detectors and the sensitivity to hydrogen gas was investigated in dependence on the active area, the Pt thickness and the operating temperature for 1 vol.% hydrogen in synthetic air.
Abstract: The performance of Pt/GaN Schottky diodes with different thickness of the catalytic metal were investigated as hydrogen gas detectors. The area as well as the thickness of the Pt was varied between 250 μm × 250 μm and 1000 μm × 1000 μm, 8 and 40 nm, respectively. The sensitivity to hydrogen gas was investigated in dependence on the active area, the Pt thickness and the operating temperature for 1 vol.% hydrogen in synthetic air. We observed a significant increase of the sensitivity and a decrease of the response and recovery times by increasing the temperature of operation to about 350 °C and by decreasing the Pt thickness down to 8 nm. Electron microscopy of the microstructure showed that the thinner Platinum had a higher grain boundary density. The increase in sensitivity with decreasing Pt thickness points to the dissociation of molecular hydrogen on the surface, the diffusion of atomic hydrogen along the Platinum grain boundaries and the adsorption of hydrogen at the Pt–GaN interface as a possible mechanism of sensing hydrogen by Schottky diodes.
72 citations
TL;DR: In this paper, the influence of the annealing temperature on film crystallinity and on optical absorption was studied, and it was shown that increased crystallinity leads to an increased optical absorption.
63 citations
TL;DR: A detailed analysis of the dielectric function for wurtzite InN as well as for In-rich InAIN alloys is presented in this article, covering the energy range from 0.72 up to 9.5 eV, were obtained by ellipsometric studies of an (1120) a-plane InN film and low carrier density (0001) c-plane films.
Abstract: A detailed analysis of the dielectric function (DF) for wurtzite InN as well as for In-rich InAIN alloys is presented. The experimental_data, covering the energy range from 0.72 up to 9.5 eV, were obtained by ellipsometric studies of an (1120) a-plane InN film and low carrier density (0001) c-plane films. Model calculations of the imaginary part of the DF around the band gap provide direct insight how to determine the energetic position of the Fermi energy from the experimental results. Then, taking into account both, band gap renormalization and Burstein-Moss shift, the values of the gaps at zero carrier density are calculated. The dependence of the InAIN band gap on the alloy composition is described by a bowing parameter of 4.0 eV. The a-plane film exhibits a characteristic optical anisotropy below 1 eV which is attributed to the polarization dependence of transition probabilities from the three valence bands at the Γ point of the Brillouin zone into the conduction band. The splitting of 25 meV between the absorption edges for the two polarization directions can be well explained by a crystal field energy of 19 (24) meV if a calculated spin-orbit energy of 13 (5) meV is assumed. All results emphasize a band gap value of wurtzite InN of about 0.68 eV. By fitting the third derivatives of the dielectric function up to 9.5 eV we determine the compositional dependences of the transition energies for at least three critical points of the band structure.
62 citations
TL;DR: In this paper, a model for the influence of different contributions to the high electron concentration in dependence on the film thickness of state-of-the-art InN layers grown by molecular-beam epitaxy is proposed.
Abstract: A model for the influence of different contributions to the high electron concentration in dependence on the film thickness of state-of-the-art InN layers grown by molecular-beam epitaxy is proposed. Surface accumulation has a crucial influence for InN layers 10μm.
53 citations
TL;DR: In this article, the formation of a dislocation network in 2H-InN epilayers during molecular beam epitaxy was studied by reflection high-energy electron diffraction, transmission electron microscopy, and high resolution x-ray diffraction.
Abstract: The strain-relaxation phenomena and the formation of a dislocation network in 2H‐InN epilayers during molecular beam epitaxy are reported. Plastic and elastic strain relaxations were studied by reflection high-energy electron diffraction, transmission electron microscopy, and high resolution x-ray diffraction. Characterization of the surface properties has been performed using atomic force microscopy and photoelectron spectroscopy. In the framework of the growth model the following stages of the strain relief have been proposed: plastic relaxation of strain by the introduction of geometric misfit dislocations, elastic strain relief during island growth, formation of threading dislocations induced by the coalescence of the islands, and relaxation of elastic strain by the introduction of secondary misfit dislocations. The model emphasizes the determining role of the coalescence process in the formation of a dislocation network in heteroepitaxially grown 2H‐InN. Edge-type threading dislocations and dislocati...
46 citations
TL;DR: In this paper, an enhancement of the quality factor by increasing the resonant frequency using strained resonator structures is proposed, which is the result of the thermal mismatch between heteroepitaxial SiC or AlN layers and the silicon substrates.
Abstract: A pulsed mode magnetomotive operation of micro- and nanoelectromechanical devices in air is demonstrated, where viscous damping determines the quality factor of the device. An enhancement of the quality factor by increasing the resonant frequency using strained resonator structures is proposed. Internal strain is the result of the thermal mismatch between heteroepitaxial SiC or AlN layers and the silicon substrates. Comparing unstrained and strained resonators, an increase of the quality factor by one order of magnitude from about 30 to 300 was achieved. This increase will improve the sensing performance of such resonant structures for an operation in ambient environment.
43 citations
TL;DR: In this paper, the energy shift of the In MNN peak was estimated during the depth profiling in order to obtain information about the position of the Fermi level, while the surface is clearly n-type.
Abstract: Non intentionally doped and Mg-doped InN layers were analyzed by sputter depth profiling in an Auger electron spectroscopy (AES) equipment and by Ultra-violet photoelectron spectroscopy (UPS). On the surface of both types of layers a high concentration of oxygen and a strong accumulation of electrons was observed, however, in contrast to the undoped layers the conductivity profile of Mg doped InN shows a strong discontinuity close to the surface. The depth of this discontinuity strongly depends on the oxygen concentration. The energy shift of the In MNN peak was estimated during the depth profiling in order to obtain information about the position of the Fermi level. In the bulk of the Mg-doped InN the In MNN peak shift of about 0.15 eV demonstrates the influence of the Mg on the Fermi level, while the surface is clearly n-type. A strong shift of the Fermi level close to the surface was observed, which might be attributed to the formation of In 2 O 3 . By the combination of AES and UPS a model for the band bending is proposed, which demonstrates that Mg doping indeed can compensate the n-type conductivity in the bulk and is therefore a prospective candidate to achieve p-type doping in InN.
TL;DR: In this paper, the optical characterization of Si-doped AlN layers grown by plasma-assisted molecular-beam epitaxy was reported, which revealed a sharp band-edge line and two broadbands around 4.4eV and 3.6eV.
Abstract: We report on the optical characterization of highly Si-doped AlN layers grown by plasma-assisted molecular-beam epitaxy. Cathodoluminescence spectra reveal a sharp band-edge line and two broadbands around 4.4eV and 3.6eV. Near-band-edge luminescence remains intense at room temperature, and shows a systematic redshift with increasing Si concentration. Regarding the low-energy bands, we observe a redshift of the emission around 3.6eV with increasing Si concentration accompanied by an increase in relative intensity compared to the 4.4eV band.
TL;DR: In this article, the authors showed that at low temperatures below 1000 °C the Kirkendall effect during the alloying of aluminium and the catalyst resulted in the formation of three-dimensional nanostructures like lamellas and nano flowers.
Abstract: AlN nanowires with a diameter of 20 nm were grown stochastically by the vapour-liquid-solid (VLS) method. At low temperatures below 1000 °C the Kirkendall effect during the alloying of aluminium and the catalyst resulted in the formation of three-dimensional nanostructures like lamellas and nano flowers. The high temperatures above 1000 °C, which are necessary to grow the nanowires complicate the control of their formation. Small catalyst droplets of 20 nm diameter are not stable due to their evaporation. Thus, in contrast to the classical approach to grow a single nano wire out of one droplet, we grew dense networks of nanowires inside larger 3D structures with diameters up to 5 µm. Depending on the growth temperature and the droplet geometry the nanowires inside of these networks are connected by angles of 90° (“cubic”) or 120° (“hexagonal”). (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
TL;DR: In this article, the performance of hydrogen-gas detectors based on Pt-GaN Schottky diodes with 24-nm-thick Pt contact was investigated, and it was shown that the observed current increase is due to a decrease in the effective barrier height (BH) through a decrease of the Pt work function upon absorption of hydrogen.
Abstract: The performance of hydrogen-gas detectors based on Pt-GaN Schottky diodes with 24-nm-thick Pt contact was investigated. Current-voltage (I-V) Characteristics were measured in two ambients (e.g., synthetic air (20% O2 in N2) and 1-vol.% H2 in synthetic air) at different temperatures. The forward current of the diodes is found to increase significantly upon introduction of H2 into the synthetic air ambient. Analysis of the I-V characteristics as a function of temperature demonstrated that the observed current increase is due to a decrease in the effective barrier height (BH) through a decrease in the Pt work function upon absorption of hydrogen. The decrease in the BH was measured as high as 30 and 152 meV at 25 degC and at 280 degC, respectively, upon introduction of H 2 into the ambient. The changes in the BH were completely reversible upon restoration of the synthetic air ambient. The sensitivity to the hydrogen gas was investigated in dependence on the operating temperature for 1-vol.% hydrogen in synthetic air. The changes in the forward bias at a constant current density of 3.2 A/cm2 was 90 and 330 mV at 25 degC and at 310 degC, respectively, upon introduction of 1-vol.% H2 into the ambient. Additionally, a significant increase in the sensitivity and a decrease in the response and recovery times have been observed after increasing the operating temperature up to ~310degC
TL;DR: In this paper, the authors investigated the influence of the AlN material quality on the etching rate in KOH-based solutions and showed that high quality MBE-AlN could be etch anisotropic with a preferred lateral component in [110] direction at 60 °C while the polycrystalline AlN layers were etched isotropic and homogenously already at room temperature.
Abstract: In this work we investigated the influence of the AlN material quality on the etching rate in KOH-based solutions. Thus, AlN layers were deposited by three different methods on sapphire and silicon substrates (i) by metal organic chemical vapor deposition (MOCVD), (ii) by molecular beam epitaxy (MBE), and (iii) by reactive sputter deposition. The etch rate is strongly dependent on crystal quality and etch temperatures. The high quality MBE-AlN could be etch anisotropic with a preferred lateral component in [110] direction at 60 °C while the polycrystalline AlN layers we etched isotropic and homogenously already at room temperature. The wet chemical etching in KOH solution is mainly an etching along defects and grain boundaries. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
TL;DR: In this paper, an alternative route to improve the epitaxial growth of 3C-SiC(100) on Si(100), which consists in covering the silicon wafers with germanium prior to the carbonization step of the silicon substrate, was developed.
Abstract: An alternative route to improve the epitaxial growth of 3C-SiC(100) on Si(100) was developed. It consists in covering the silicon wafers with germanium prior to the carbonization step of the silicon substrate. Transmission electron microscopy and μ-Raman investigations revealed an improvement in the residual strain and crystalline quality of the grown 3C-SiC layers comparable to or better than in the case of 3C-SiC grown on silicon on insulator substrates. These beneficial effects were reached by using a Ge coverage in the range of 0.5–1 monolayer.
TL;DR: In this article, the piezoresponse force microscopy and an interferometric technique were used to measure the properties of Wurtzite AlN thin films on Si (111) substrates by reactive DC-sputtering and by metalorganic chemical vapor deposition.
Abstract: Piezoelectric properties of aluminium nitride thin films were measured using both, the piezoresponse force microscopy and an interferometric technique. Wurtzite AlN thin films were prepared on Si (111) substrates by reactive DC-sputtering and by metalorganic chemical vapor deposition. Direct measurements of the inverse piezoelectric effect in the picometer range showed that the acceptable tolerance in the crystal orientation is much larger for MEMS applications than expected previously. The value of the piezoelectric coefficient d33 for the prepared AlN thin films was determined to be 5.36 ± 0.25 pm/V for highly textured as well as for polycrystalline thin films with a (002) preferential orientation. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
TL;DR: In this article, the beam quality parameter M2 and output power were investigated for a single quantum well InGaAs/AlGaAs device emitting at 976 nm by molecular beam epitaxy.
Abstract: Tapered diode lasers combine high output power and a beam quality near to the diffraction limit resulting in very high
brightness. Therefore, they are finding use in a wide range of applications today, such as pumping of rare-earth-doped
fibre amplifiers, tunable frequency doubling of diode lasers for blue-green outputs, and non linear spectroscopy. Due to
increasing brightness and lifetime tapered lasers even become attractive for material processing and for telecom
applications like pumping of Er-doped fiber amplifiers or raman amplifiers.
In order to further enhance the brightness of tapered diode lasers the output power has to be increased while
simultaneously the beam quality has to be kept near the diffraction limit. For this purpose we have grown low modal
gain, single quantum well InGaAs/AlGaAs devices emitting at 976 nm by molecular beam epitaxy. The lateral design of
the investigated laser diodes consists of a tapered section and a ridge-waveguide section. Since it has been shown by
previous simulations and experiments that longer tapered sections allow higher output power with unchanged beam
quality, we use tapered section lengths of 2000 μm, 3000 μm and 4000 μm. The beam quality parameter M2 and output
powers as well as the nearfields of the different structures were carefully investigated. For longer devices we reach an
optical output power of more than 10 W per single emitter in continuous wave mode (cw) without any distinct thermal
rollover.
TL;DR: In this paper, the impact of Si impurities on the structural properties of AlN, grown by plasma-assisted molecular-beam epitaxy on c-plane sapphire is studied.
Abstract: The impact of Si impurities on the structural properties of AlN, grown by plasma-assisted molecular-beam epitaxy on c-plane sapphire is studied. Under nitrogen-rich growth conditions silicon can be homogeneously incorporated up to Si concentrations of [Si]=5.2×1021 cm−3. The presence of silicon on the surface during the growth process is demonstrated to be beneficial for the surface morphology and the structural properties of the AlN films. For [Si] up to (5±3)×1020 cm−3, this surfactant behavior results in a decrease of the surface roughness from 8 nm for undoped layers grown in a nitrogen-rich regime to less than 1 nm. In addition, high resolution x-ray diffraction studies reveal an increase of the average lateral crystal size from 300 nm to more than 1 μm and a simultaneous decrease of the screw dislocation density from 3.8×108 cm−2 for (comparably) weakly doped samples to 2×107 cm−2. At the same [Si] the heterogeneous stress shows a minimum of less than 50 MPa and drastically increases for higher [Si]...
TL;DR: In this paper, micro and nanomechanical AlN and 3C-SiC resonator beams with resonant frequencies between 20 kHz and 2 MHz, depending on the resonator geometry, have been realized and characterized at ambient conditions.
Abstract: Micro- and nanomechanical AlN and 3C-SiC resonator beams with resonant frequencies between 20 kHz and 2 MHz, depending on the resonator geometry, have been realized and characterized at ambient conditions. Up to 200 nm thin epitaxial group III-nitrides and SiC layers were grown on silicon (111) and (100) oriented substrates, respectively. The beams were dry-etched by an electron cyclotron resonance (ECR) plasma and an inductive coupled plasma (ICP) technique. The freestanding resonator bars have dimensions in the sub-μm to nm-range. The operation principle based on the known magneto motive actuation and a thin conductive metal layer on top of the resonator realizes the detection. The main fabrication steps of the resonator beams are presented. The resonant frequencies, the quality factors of the MEMS and NEMS are investigated in dependence on the geometry and the residual strain in the epitaxial layers.
19 Apr 2006
TL;DR: In this article, a novel semiconductor material, called InN, was used as THz surface emitter and the material was irradiated with fs-laser pulses at 1060 nm and 800 nm and the emitted ultrashort THz pulses were measured by phase sensitive detection.
Abstract: InN, a novel semiconductor material, is used as THz surface emitter. The material is irradiated with fs-laser pulses at 1060 nm and 800 nm and the emitted ultrashort THz pulses are measured by phase sensitive detection. Pulsforms, amplitudes and spectra are compared to the THz emission of p-doped InAs, the standard material for THz surface emission.
TL;DR: In this article, the optical and electronic properties of diamond-like amorphous carbon (a-C:H and a-C 1−x6:H) films were studied by photothermal deflection spectroscopy and the spectral photocurrent method.
Abstract: Optical and electronic properties of plasma enhanced chemical vapor deposited diamondlike amorphous carbon (a-C:H and a-C1−xSix:H) films were studied by photothermal deflection spectroscopy and the spectral photocurrent method. Two different regimes of photoconductivity in the diamondlike carbon films have been detected. The carrier mobility due to the σ-σ* bands is found to be 1–2 orders of magnitude higher than that due to the π-π* transitions. The σ-σ* mobility edge of diamondlike hydrogenated amorphous carbon is equal to 5.3±0.1eV and appears to be closely correlated with the band gap of diamond. We have also shown that the mobility edge in diamondlike amorphous carbon is not associated with its band gap. The optical band gap of diamondlike amorphous carbon films depends on the sp2 phase content as already reported by Robertson [Phys. Rev. B 53, 16302 (1996)]. However, the mobility edge is not influenced by either the Si doping level, the film thickness, or the ion impact energy during deposition.
TL;DR: In this article, the results obtained by various procedures applied to LVV and KLL Auger peaks of silicon (Si) for determining the Auger current were compared, and it was shown that the results for the content of Si in SiC can be achieved by the method in which the area below the Augerspeak in the direct N (E ) spectrum after subtracting the background of inelastically scattered Auger electrons of the given peak and of the so-called true secondary electrons.
TL;DR: In this paper, it was found that both surfaces still change their properties in the first days/hours after the modification, which are explained by the different chemical-bond structures of the various fluorocarbon (FC) films.
Abstract: Modifications of AlGaN surfaces have been carried out in order to tune their wetting properties. A hydrophilic surface is achieved by a wet and dry thermal oxidation, whereas the deposition of fluorocarbon (FC) layers leads to a passivation with a hydrophobic behavior. It was found that both surfaces still change their properties in the first days/hours after the modification. For the FC layers, differences are observed in dependence of the deposition method, which are explained by the different chemical-bond structures of the various FC films
TL;DR: In this article, the work function difference between Ga-face and N-face GaN is found to be 0.25eV. This difference is caused by a surface band bending.
Abstract: Lateral polarity heterostructure (LPH) was grown with adjacent Ga- and N-face domains in order to invert the polarity of the crystal within a periodicity of a few microns. In this study we focus on the analysis of these LPH by Auger electron spectroscopy (AES). Because of the relationship between the Auger electron energy and the Fermi level, AES is a suitable method to identify the domains of a lateral polarity heterostructure. In addition, we discuss the possibility of determining the work function difference of Ga- and N-face GaN. This difference in the work function between Ga-face and N-face GaN is found to be 0.25eV. This difference is caused by a surface band bending.
TL;DR: In this paper, the authors calculated the imaginary part of the dielectric function close to and above the absorption edge of wurtzite ZnO and found three main contributing mechanisms: optical transitions involving discrete exciton states, excitonically enhanced interband optical transitions, and exciton-phonon complexes.
TL;DR: In this article, the growth of fullerene films on the InP(001)-(2? 4) surface and the formation of the C60/InP(1)-2 2 2 4 interface were studied by x-ray photoelectron spectroscopy, ultraviolet photo electromagnetic resonance (UVR) and low energy electron diffraction.
Abstract: The growth of fullerene films on the InP(001)-(2 ? 4) surface and the formation of the C60/InP(001)-(2 ? 4) interface were studied by x-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, electron energy loss spectroscopy and low energy electron diffraction. C60 adsorption causes weak (~0.15?eV) upward band bending at the interface. Thick C60 films form an fcc (111) structure on the InP(001) surface. The (2 ? 4) reconstruction is preserved beneath the C60 film. The photoelectron measurements yield a valence band discontinuity of 0.88 ? 0.20?eV at the C60/InP(001)-(2 ? 4) interface.
TL;DR: In this article, the authors analyzed the spectral response as a function of the applied voltage in the frameworks of the space-charge limited current model and found that the main source of PPC is traps above the Fermi level.
Abstract: The responsivity and the decay time of AlGaN solar blind UV-detectors have been studied. The photodetectors have shown a good spectral responsivity in a narrow spectral range (220 < A < 300 nm) and a short time response with the best estimated characteristic time constant of r∼ 30 ms measured at room temperature. Possible mechanisms responsible for the persistent photoconductivity (PPC) effect in Al x Ga 1-x N (x = 0.51) are analyzed. A shape of the spectral response as a function of the applied voltage is analyzed in the frameworks of the space-charge limited current model. It has been shown that the main source of PPC is traps above the Fermi level. PPC occurs when the density of free carriers equals the density of the traps. The model attributing PPC to the spatial separation of the photoexcited electrons and holes by the macroscopic potential barrier is not supported by our photocurrent studies.
TL;DR: In this article, the influence of different contributions to the high electron concentration in state-of-the-art InN layers grown by molecular-beam epitaxy is investigated, and it is shown that surface accumulation has a crucial influence for thin InN-Schichten 10 μm.
Abstract: The influence of different contributions to the high electron concentration in state-of-the-art InN layers grown by molecular-beam epitaxy is investigated. Surface accumulation has a crucial influence for thin InN layers 10 μm.
Quellen der n-Leitfahigkeit in undotiertem InN
Der Einfluss verschiedener Mechanismen auf die hohe Elektronenkonzentration in durch Molekularstrahlepitaxie gewachsenen state-of-the-art InN-Schichten wird untersucht. Die Oberflachenakkumulation von Elektronen dominiert die Leitfahigkeit fur InN-Schichten 10 μm dominieren.
TL;DR: In this article, the imaginary part of the dielectric function of zinc-blende III-V semiconductors in the vicinity of the direct absorption edge was calculated using the $kp$ model of the band structure and the Elliott's theory of excitonic absorption.
Abstract: The imaginary part of the dielectric function of zinc-blende III-V semiconductors in the vicinity of the direct absorption edge was calculated using the $kp$ model of the band structure and the Elliott's theory of excitonic absorption. From comparison to the experimental data, we found that the excitonic contribution is to be weighted in favor of the band-to-band transitions by a factor which lies between zero and unity depending on the semiconductor and temperature. It was shown that the ratio ${\ensuremath{\lambda}}_{p}∕{a}_{x}$, where ${\ensuremath{\lambda}}_{p}$ and ${a}_{x}$ are the phonon mean free path and the exciton Bohr radius, respectively, determines the intensity of the excitonic absorption.