Observation of spatially-indirect transition and accurate determination of band offset ratio by excitation spectroscopy on GaAs/AlGaAs quantum wells lightly doped with Be accepters

We report on the observation of a spatially-indirect transition in the photoluminescence (PL) excitation spectra of Be-doped GaAs/AlGaAs quantum wells (QWs). Using this spatially-indirect transition we determine the band offset ratio accurately. With Be accepters in the QW, free-to-bound and bound-exciton transitions show up in PL in addition to free-exciton transition. As we vary the excitation photon energy, we find a sharp intensity changeover between the free-to-bound and the excitonic transitions at a certain photon energy. A systematic investigation of this energy as a function of the well width shows that this feature corresponds to the onset of a spatially-indirect transition from the valence-band top of the barrier to the n = 1 conduction subband in the QW. Based on the fact that the energy of this barrier-to-well transition critically depends on the valence-band offset, we have determined the conduction-band offset ratio as Q_c = 0.62 with accuracy better than ΔQ_c = ±0.01.     

Luminescence of GaN single crystals prepared by heating a Ga melt in Na–N2 atmosphere

Colorless transparent prismatic crystals (0.5‐2.0 mm long) and hopper crystals (1.0‐2.5 mm long) of GaN were prepared by heating a Ga melt at 800°C in Na vapor under N₂ pressures of 7.0 MPa for 300 h. The photoluminescence (PL) spectrum of a prismatic crystal at 4 K showed the emission peaks of neutral donor‐bound exciton (D⁰‐X) and free exciton (X_A) at 3.472 eV and 3.478 eV, respectively, in the near band edge region. The full‐width at half‐maximum (FWHM) of (D⁰‐X) peak was 1.9 meV. The emission peaks of a donor–acceptor pair transition (D⁰‐A⁰) and its phonon replicas were observed in a lower energy range (2.9‐3.3 eV). The emission peaks of the D⁰‐A⁰ and phonon replicas were also observed in the cathodoluminescence (CL) spectrum at 20 K. The (D⁰‐X) PL peak of a hopper crystal at 4 K was at 3.474 eV (2.1 meV higher), having a FWHM of 6.1 meV which was over 3 times larger than that of the prismatic crystal. A strong broad band with a maximum intensity around 1.96 eV was observed for the hopper crystals in the CL spectrum at room temperature. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Photoelectronic and electrical properties of CuIn5S8 single crystals

To identify the impurity levels in CuIn₅S₈ single crystals, the dark electrical conductivity and photoconductivity measurements were carried out in the temperature range of 50–460 K. The data reflect the intrinsic and extrinsic nature of the crystals above and below 300 K, respectively. Energy band gaps of 1.35 and 1.31 eV at 0 K and 300 K, were defined from the dark conductivity measurements and the photocurrent spectra, respectively. The dark and photoconductivity data in the extrinsic temperature region reflect the existence of two independent donor energy levels located at 130 and 16 meV. The photocurrent‐illumination intensity dependence (F) follows the law I_phαF^γ, with γ being 1.0, 0.5 and 1.0 at low, moderate and high intensities indicating the domination of monomolecular, bimolecular and strong recombination at the surface, respectively. In the intrinsic region and in the temperature region where the shallow donor energy level 16 meV is dominant, the free electron life time, τ_n, is found to be constant with increasing F. In the temperature region 140 K < T < 210 K, the free electron life time increases with increasing illumination intensity showing the supralinear character. Below 140 K, τ_n decrease with decreasing illumination intensity. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Temperature dependence of photoluminescence from ordered GaInP2 epitaxial layers

The temperature behavior of the integrated intensity of photoluminescence (PL) emission from ordered GaInP₂ epitaxial layer was measured at temperatures of 10 ‐ 300 K. Within this temperature range the PL emission is dominated by band‐to‐band radiative recombination. The PL intensity temperature dependence has two regions: at low temperatures it quenches rapidly as the temperature increases, and above 100 K it reduces slowly. This temperature behavior is compared with that of disordered GaInP₂ layer. The specter of the PL emission of the disordered layer has two peaks, which are identified as due to donor‐accepter (D‐A) and band‐to‐band recombination. The PL intensity quenching of these spectral bands is very different: With increasing temperature, the D‐A peak intensity remains almost unchanged at low temperatures and then decreases at a higher rate. The intensity of the band‐to‐band recombination peak decays gradually, having a higher rate at low temperatures than at higher temperatures. Comparing these temperature dependencies of these PL peaks of ordered and disordered alloys and the temperature behavior of their full width at half maximum (FWHM), we conclude that the different morphology of these alloys causes their different temperature behavior. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structural,electrical and optical properties of Ge implanted GaSe single crystals grown by Bridgman technique

Structural, optical and electrical properties of Ge implanted GaSe single crystal have been studied by means of X‐Ray Diffraction (XRD), temperature dependent conductivity and photoconductivity (PC) measurements for different annealing temperatures. It was observed that upon implanting GaSe with Ge and applying annealing process, the resistivity is reduced from 2.1 × 10⁹ to 6.5 × 10⁵ Ω‐cm. From the temperature dependent conductivities, the activation energies have been found to be 4, 34, and 314 meV for as‐grown, 36 and 472 meV for as‐implanted and 39 and 647 meV for implanted and annealed GaSe single crystals at 500°C. Calculated activation energies from the conductivity measurements indicated that the transport mechanisms are dominated by thermal excitation at different temperature intervals in the implanted and unimplanted samples. By measuring photoconductivity (PC) measurement as a function of temperature and illumination intensity, the relation between photocurrent (I_PC) and illumination intensity (Φ) was studied and it was observed that the relation obeys the power law, I_PC αΦⁿ with n between 1 and 2, which is indication of behaving as a supralinear character and existing continuous distribution of localized states in the band gap. As a result of transmission measurements, it was observed that there is almost no considerable change in optical band gap of samples with increasing annealing temperatures for as‐grown GaSe; however, a slight shift of optical band gap toward higher energies for Ge‐implanted sample was observed with increasing annealing temperatures. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Photoluminescence in doped and annealed GeSe2 glass

Photodoping of GeSe₂ glass with Ag, Zn, In and Sn shifted the centre of the photoluminescence (PL) band towards lower energies and gave rise to a decrease in its half width, but no new band appeared. The excitation spectrum at 77 K and the temperature dependence of the PL intensity of Ag-doped samples were measured in the temperature range (77–280) K. An activation energy of (253 ± 10) meV for non radiative transitions was foundat T > 210 K which is smaller than that measured in undoped GeSe₂ glass. Thermal annealing shifted the peak of the PL band towards higher energies but had no effect on its half width.     

Einfluß heißer Ladungsträger auf die Donator-Akzeptor-Paar-Rekombination

The capture cross section σ = AR² usually applied in calculations concerning the donoracceptor pair recombination has been used in a modified form, taking into account a contribution of resonant capture of hot carriers. To study the influence of highly excited electrons the donor-acceptor pair transition serves as suitable probe because of the appreciable peak energy shift of the main emission band with increasing excitation. It is shown, that resonant capture of hot carriers contributes to this peak energy shift.     

Effektive Masse und Beweglichkeit der Löcher in p-ZnSiAs2

The density of states effective mass of the valence band and impurity parameters in p-ZnSiAs₂ were determined from the temperature dependence of the Hall coefficient. The density of states effective mass of holes in ZnSiAs₂ is (0.21 ± 0.03) m₀. Three acceptor levels located at 15 meV, 40 meV and 200 meV upper the valence band edge were found. A mobility analysis was carried out taking into account the interaction of the current carriers with the large number of longitudinal optical branches existing in chalcopyrite structure. The experimental values are in good accordance with the theoretical mobility curves in the temperature range from 77 K up to 600 K.     

Photoconduction of glasses in the TeSeSb system

The photoconductivity of bulk glasses of the TeSeSb system is measured as a function of light intensity and photon energy. The relative sensitivity (ΔI/I_d) has linear and square-root dependences on light intensity in low and high illumination intensities, respectively, and is nearly proportional to the square-root of the resistivity at room temperature. The spectral response of photoconductivity, which is calculated by taking into account the effect of surface recombination of carriers, agrees qualitatively with the experimental results. The experimentally determined broad spectral response suggests the presence of band tails below the conduction band and above the valence band. The large residual dark conductivity in the decay response is associated with the presence of many deep trapping centers.     

Infrared photoluminescence from TlGaS2 layered single crystals

Photolimuniscence (PL) spectra of TlGaS₂ layered crystals were studied in the wavelength region 500‐1400 nm and in the temperature range 15‐115 K. We observed three broad bands centered at 568 nm (A‐band), 718 nm (B‐band) and 1102 nm (C‐band) in the PL spectrum. The observed bands have half‐widths of 0.221, 0.258 and 0.067 eV for A‐, B‐, and C‐bands, respectively. The increase of the emission band half‐width, the blue shift of the emission band peak energy and the quenching of the PL with increasing temperature are explained using the configuration coordinate model. We have also studied the variations of emission band intensity versus excitation laser intensity in the range from 0.4 to 19.5 W cm^‐2. The proposed energy‐level diagram allows us to interpret the recombination processes in TlGaS₂ crystals. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

New garnet crystal potentially suited for mini-laser devices of 1.5-μm spectral range

In Er:doped crystals, the 1.5-μm (⁴I_13/2–⁴I_15/2) transition is of negligibly small intensity. To intensify this transition, the (Gd,Y)₃(Ga,Sc)₅O₁₂ host crystal has been chosen as a basic medium. The single crystal garnet films with thickness up to 18-μm were grown using the method of liquid-phase epitaxy on Gd₃Ga₅O₁₂ substrates. The 20-at.% maximal concentration of Er³⁺-ions was achieved without luminescence quenching. The up-conversion processes were neutralized by the addition of an Fe-ions sensitizer. At the same level of absorbed pumping power, the luminescence intensity at the 1.5-μm band for the Er:Fe:doped crystal was approximately one to two orders of magnitude higher than that for traditional content. Heavily doped crystals demonstrated broadening of the luminescence band up to 300 nm.     

Band tail conduction in Zn-doped GaAs

The electrical parameters of Zn-doped GaAs liquid phase epitaxial layers are determined in the temperature range from 77 to 300 K by Hall effect and resistivity measurements. An analysis of the experimental data yields an ionization energy of 30.4 meV for the Zn acceptor in GaAs in the dilute limit of acceptor concentration. For the Mott transition a critical hole concentration of about 6 · 10¹⁷ cm⁻³ is estimated. It is found that the temperature dependence of the electrical parameters is essentially influenced by band tail conduction effects at doping levels above the Mott transition.     

Evaluation for photo-induced changes of photoluminescence and optical energy gap in amorphous Se100−xGex (x=5, 25 and 33) thin films

We have investigated photo-induced changes of photoluminescence (PL) and optical energy gap (E_OP) in amorphous Se_100−xGe_x (x=5, 25 and 33) thin films from illumination by HeCd laser light (hν=3.81 eV). The E_OP is obtained by a linear extrapolation of (αhν)^1/2 vs. hν to the energy axis. The optical absorption coefficient (α) is calculated from the extinction coefficient k measured in the wavelength range of 290-900 nm. Although the values of ΔE_OP are very different, all films exhibit a photo-induced photodarkening (PD) effect that is a red shift of E_OP. In particular, ΔE_OP in a-Se₇₅Ge₂₅ thin film exhibits the largest value (i.e., ΔE_OP∼40 meV for a-Se₉₅Ge₅, ΔE_OP∼200 meV for a-Se₇₅Ge₂₅, ΔE_OP∼130 meV for a-Se₆₇Ge₃₃). PL spectra in a-SeGe by hν_HeCd have no Stokes shift (SS). Although the PL intensity in the Se_100−xGe_x system is very small, the PL spectra (three-trial average) show a dependence on both composition and illumination time. The intensities of five band peaks (I₁-I₅) observed in the range of 1-2 eV increase with an increase of Ge content. The intensity of I₂ peak (the strongest intensity) increases with increasing illumination time, while the peak-energy position decreases. This is evidence of PD. A decrease in PL peak-energy position with Ge contents shows a similar tendency to that in E_OP. That is, the ΔE(I₂) is 25.2, 45.0 and 44.5 meV for a-Se₉₅Ge₅, a-Se₇₅Ge₂₅ and a-Se₆₇Ge₃₃, respectively. We propose in this paper a triple-well potential model that can universally explain the energy-induced phenomena in the a-SeGe, such as the PD and thermal bleaching, the native charged-defect generation and the no-SS PL, etc.     

Comparative optical absorption and photoreflectance study of n-type CuInSe2 single crystals

Optical absorption spectra in the photon energy range from 0.4 to 1.2 eV and photoreflectance spectra in the range of the fundamental edge are measured on n-type CuInSe₂ single crystals. Photoreflectance spectroscopy yields the true gap energy while the near-edge absorption spectra are dominated by acceptor–to–conduction band transitions, the acceptor ionisation energy being about 80 meV. Based on intrinsic defect chemistry considerations this acceptor is ascribed to copper vacencies.     

Improvement of the electrical properties of Se3Te1 thin films by In additions

The electronic and photoconductivity properties of semiconducting chalcogenide glasses have been largely stimulated by attractive micro-electronic device applications. The present paper aims to study the effect of In additions on the steady state and transient photoconductivity of amorphous In_x(Se₃Te₁)_100 ? x (0 ≤ x ≤ 10 at.%) chalcogenide films. It was found that, the Indium additions lead to the decrease of both the activation energies (ΔE_dc in the dark and ΔE_ph for the photoelectrical conduction) and the optical band gap E_g that improved the electrical properties of these films. The photoconductivity increases while photosensitivity changes from 8.73 to 7.18 with the increase of In content. The exponential dependence of photocurrent on the light intensity suggests that, the recombination mechanism in these films is due to bimolecular recombination. The transient photoconductivity measurements stated that, the carrier lifetime decreased by the increase of the light intensity and In content. The obtained results were discussed in terms of the width of localized states (Mott and Davis model) and the chemical-bond approach.     

Electrical,optical and photoconductive properties of Poly(dibenzo‐18‐crown‐6)

To investigate the energy levels, absorption bands, band gap, dominant transport mechanisms, recombination mechanisms and the free carrier life time behavior of poly‐dibenzo‐18‐crown‐6, poly‐DB18C6, films, the dark electrical conductivity in the temperature range of 200‐550 K, the absorbance and photocurrent spectra, the photocurrent –illumination intensity and time dependence at 300 K were studied. The dark electrical conductivity measurements revealed the existence of three energy levels located at 0.93, 0.32 and 0.76 eV below the tails of the conduction band. The main transport mechanism in the dark was found to be due to the thermal excitation of charge carriers and the variable range hopping above and below 260 K, respectively. The photocurrent and absorbance spectra reflect a band gap of 3.9 eV. The photocurrent ‐illumination intensity dependence reflects the sublinear, linear and supralinear characters indicating the decrease, remaining constant and increase in the free electron life time that in turn show the bimolecular, strong and very strong recombination characters at the surface under the application of low, moderate and high illumination intensity, respectively. A response time of 25.6 s was calculated from the decay of I_ph‐time dependence. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Photoluminescence Spectra of GaS0.75Se0.25 Layered Single Crystals

Photoluminescence (PL) spectra of GaS_0.75Se_0.25 layered single crystals have been studied in the wavelength region of 500‐850 nm and in the temperature range of 10‐200 K. Two PL bands centered at 527 ( 2.353 eV, A‐band) and 658 nm (1.884 eV, B‐band) were observed at T = 10 K. Variations of both bands have been studied as a function of excitation laser intensity in the range from 8 × 10^‐3 to 10.7 W cm^‐2. These bands are attributed to recombination of charge carriers through donor‐acceptor pairs located in the band gap. Radiative transitions from shallow donor levels located 0.043 and 0.064 eV below the bottom of conduction band to acceptor levels located 0.088 and 0.536 eV above the top of the valence band are suggested to be responsible for the observed A‐ and B‐bands in the PL spectra, respectively.     

Relationship between defects and optical properties in Er-doped GaN

The density of the vacancy-type defect in Er doped GaN was measured by positron annihilation spectrometry (PAS) and the correlation between the intensity of the Er-related luminescence was studied. A luminescence peak at 558 nm originating from ⁴S_3/2 to ⁴I_15/2 transition of Er³⁺ was observed in Er-doped GaN. The intensity of the luminescence increased with increasing Er concentration and showed the maximum with the Er concentration of around 4.0 at%. The PAS measurements showed that the vacancy-type defect density increased with increasing Er concentration up to 4 at%, and around 4 at% of Er, the formation of defect complex such as V_Ga–V_N was suggested. The contribution of the defect to the radiative recombination of intra-4f transition of Er is discussed.     

Frequency resolved photoconductivity of bulk CdS single crystals

Photoconductivity measurements in CdS single crystal showed that a.c. photoconductivity decays as ω^−s, with s value - {1 + α} for ω > 1/τ and - {1–α} for ω < 1/τ. At low frequency values, recombination might be controlled by monomolecular behaviour. As the frequency increases bimolecular behaviour, in shallow states, dominates. Direct recombination is found to be weak dependent on both the intensity and applied voltage. However when band tail recombination is dominant (at higher frequencies), strong dependence is observed.     

Luminescence of GeO2 glass, rutile-like and α-quartz-like crystals

The luminescence of GeO₂ rutile-like crystals was studied. Crystals were grown from a melt of germanium dioxide and sodium bicarbonate mixture. Luminescence of the crystal was compared with that of sodium germanate glasses produced in reduced and oxidized conditions. A luminescence band at 2.3 eV was observed under N₂ laser (337 nm). At higher excitation photon energies and X-ray excitation an additional band at 3 eV appears in luminescence. The band at 2.3 eV possesses intra-center decay time constant about 100 μs at 290 K and about 200 μs at low temperature. Analogous luminescence was obtained in reduced sodium germanate glasses. No luminescence was observed in oxidized glasses under nitrogen laser, therefore the luminescence of rutile-like crystal and reduced sodium germanate glass was ascribed to oxygen-deficient luminescence center modified by sodium. The band at 2.3 eV could be ascribed to triplet-singlet transition of this center, whereas the band at 3 eV, possessing decay about 0.2 μs, could be ascribed to singlet-singlet transitions. Both bands could be excited in recombination process with decay kinetics determined by traps, when excitation realized by ArF laser or ionizing irradiation with X-ray or electron beam. Another luminescence band at 3.9 eV in GeO₂ rutile-like crystal was obtained under ArF laser in the range 100-15 K. Damaging e-beam irradiation of GeO₂ crystal with α-quartz structure induces similar luminescence band.