Donor-acceptor recombination in short-period silicon-doped GaAs/AlAs superlattices

A low-frequency band is observed along with an exciton band in photoluminescence spectra of short-period GaAs/AlAs superlattices doped with Si in the barriers or in the barriers and wells. This band is ascribed to donor-acceptor recombination on the basis of the dependence of its frequency on the excitation intensity under cw excitation and on the time delay under pulsed excitation. Mainly type-II superlattices are investigated. The estimate E _A+E _D≈120 meV can be obtained from the peak energy of the donor-acceptor band with a very weak excitation intensity. The estimates E _A≈23 meV and E _D≈90 meV are obtained from the temperature dependence of the band intensity. It is suggested that the deep donor level is associated with a DX center in the AlAs layers. Fiz. Tekh. Poluprovodn. 32, 839–842 (July 1998)     

Impurity states in cobalt-doped silicon

Cobalt was diffused into p⁺ pn⁺ silicon structures at 900° and 1150°C for 2−4 hours followed by various quenching conditions. Four primary hole traps and two electron traps associated with cobalt in these devices were observed. The hole traps are labeled H1(E_v + 0.22 eV), H2(E_v + 0.29 eV), H3 (E_v + 0.40 eV) and H4(E_v + 0.45 eV) while the electron traps labeled E1 and E2 are located at E_c − 0.36 eV and E_c − 0.44 eV, respectively. The concentrations, thermal emission rates, and the capture cross sections for the majority carriers at these defects are reported. The behavior of these defects under heat treatment and the emergence of secondary defects, H5(E_v +0.22 eV) and H6 (E_v +0.34 eV), will be discussed.     

Time—resolved Photoluminescence Spectra of Nominally Disordered GaInP Alloy

Luminescence decay and time-resolved photoluminescence(TRPL) spectra are used for study on the transient luminescence process of the nominally disordered GaInP alloy.The luminescence decay of GaInP alloy shows the temperature and excitation-intensity dependent characters.At 77K and under high excitation intensity, the luminescence decay shows single exponential time dependence, while under low excitation intensity or at 300 K, the luminescence decay shows double exponential time dependence.The analysis indicated that this nominally disordered GaInP alloy actually exhibits a very weak degree of order.The blue-shift of PL peak is observed in the TRPL spectra at 77 K, which is derived from the transfer of the carriers from the ordered domain to the disordered region of the alloy.At 300 K, due to the thermal quenching, the transfer is too weak to be observed.However,The recombination of the carriers between the ordered domain and the disordered region is still devoted to luminesce.     

Luminescence decay kinetics in GaP:Bi and GaP:N

The lifetime of excitons bound to isoelectronic traps in GaP is studied as a function of temperature, excitation energy and intensity. The photoluminescence was excited both selectively and above the gap by using a pulsed, tunable dye laser with photon flux varying in the range of 10¹⁴–10¹⁹ photons/cm² per pulse. GaP:Bi shows a strong intensity dependence of τ(T) in the temperature range of 40–60 K due to the thermal activation of the electron. Similarly, GaP:N shows this behavior in the range of 20–100 K. In this case, two activation processes can be identified: release of the bound exciton into the free exciton band and dissociation of the exciton. The observed dependence of τ(T) on both excitation energy and intensity indicate that saturable deep traps (shunt paths) deplete the excess free carriers. These traps can be completely saturated in GaP:N while only partial saturation is achieved in GaP:Bi. The kinetic equations are written for GaP:Bi and solved numerically assuming quasiequilibrium conditions. Fitting this model to the experimental results yields the capture cross sections for carriers by Bi and by the deep traps as well as the concentration of the latter.     

Shallow acceptor levels in 4H- and 6H-SiC

Shallow impurities are the principal means of affecting the electrical properties of semiconductors in order to induce desired characteristics. They can be used to isolate a region by introducing carriers of opposite charge, or they can be used to enhance the number of carriers of a particular type. Thermal admittance spectroscopy has been used to determine the activation energies of the principal p-type dopants, Al and B, in 4H and 6H-SiC, and temperature dependent Hall effect measurements were used to study the shallow B acceptors in 6H-SiC. The accept or species B and Al occupy inequivalent lattice sites in the Si sublattice, and would be expected to exhibit distinct energy levels for each site in analogy to the well known donor energy levels of N. Activation energies for B in 6H-SiC were found to be E_h=E_v+0.27 eV, E_k1=E_v+0.31 eV, and E_k2=E_v+0.38 eV. Al acceptors in 4H-SiC were found to exhibit two energy levels at E_h=E_v+0.212 eV and E_k=E_v+0.266 eV.     

Characteristic photoluminescence band in Si-implanted SiO2 grown on Si wafer

A possible mechanism for the photoemission from Si nanocrystals in an amorphous SiO₂ matrix fabricated by ion implantation is reported. We have measured the implantation dose dependence as well as the oxidation effect of the photoluminescence behavior of Si nanocrystals in SiO₂ layers fabricated by ion implantation and a subsequent annealing step. After annealing, a photoluminescence band, peaking just below the 1.7 eV was observed. The peak energy of the photoluminescence was found to be affected by the dose of implanted Si ions, but to be independent of annealing time and excitation photon energy. We also present experimental results of an oxidation induced continuous peak energy shift of the photoluminescence peak, up to around 1.8 eV. This peak energy, however, was found to return to its previous position with re-annealing. These results indicate that, whilst the excitation photons are absorbed by Si nanocrystals, the emission is not simply due to electron–hole recombination inside the Si nanocrystals, but is related to the presence of defects, most likely located at the interface between the Si nanocrystals and the SiO₂, for which the characteristic energy levels are affected by cluster–cluster interactions or the roughness of the interface.     

Quantum efficiency and formation of the emission line in light-emitting diodes based on InGaN/GaN quantum well structures

The spectra of electroluminescence, photoluminescence, and photocurrent for the In_0.2Ga_0.8N/GaN quantum-well structures are studied to clarify the causes for the reduction in quantum efficiency with increasing forward current. It is established that the quantum efficiency decreases as the emitting photon energy approaches the mobility edge in the In_0.2Ga_0.8N layer. The mobility edge determined from the photocurrent spectra is E _me = 2.89 eV. At the photon energies hv > 2.69 eV, the charge carriers can tunnel to nonradiative recombination centers with a certain probability, and therefore, the quantum efficiency decreases. The tunnel injection into deep localized states provides the maximum electroluminescence efficiency. This effect is responsible for the origin of the characteristic maximum in the quantum efficiency of the emitting diodes at current densities much lower than the operating densities. Occupation of the deep localized states in the density-of-states “tails” in InGaN plays a crucial role in the formation of the emission line as well. It is shown that the increase in the quantum efficiency and the “red” shift of the photoluminescence spectra with the voltage correlate with the changes in the photocurrent and occur due to suppression of the separation of photogenerated carriers in the field of the space charge region and to their thermalization to deep local states.     

Thermally activated resonance tunneling in asymmetric systems of CdSe/ZnSe double quantum wells with self-assembled quantum dots

In the luminescence study of double quantum wells formed by depositing two CdSe layers with different nominal thicknesses into a ZnSe matrix, a heavy dependence of the photoluminescence spectrum on the thickness of the ZnSe barrier separating the quantum wells, the excitation photon energy, and temperature is observed. The photoluminescence spectra are studied at barrier widths of 34, 50, and 63 monolayers, excitation photon energies of 3.06, 2.71, and 2.54 eV, and temperatures T in the range of 5–200 K. Upon above- (3.06 eV) and below-barrier (2.71 eV) excitation, the photoluminescence spectrum exhibits two bands, I ₁(T) and I ₂(T), corresponding to the annihilation of excitons localized in the quantum dots of the shallow and deep quantum wells. An increase in temperature to ～50 K yields only a slight decrease in the total integrated emission intensity of both bands I _PL(T) and the intensities of each of the two bands, I ₁(T) and I ₂(T). A further increase in temperature results in substantial redistribution of the photoluminescence intensity between the two wells, which is attributed to the tunneling of excitons from the QD (quantum-dot) states of the shallow well to states of the deep well. This process is of the activation character and manifests itself as a sharp decrease in the integrated emission intensity related to the shallow quantum well, I ₁(T), and a simultaneous increase in the integrated emission intensity of quantum dots of the deep quantum well, I ₂(T). The experimentally detected effect is most profound in the range of temperatures T = 110–130 K and in the samples with a barrier thickness of 50 monolayers. It is most likely that the tunneling is of a resonance nature. This inference follows from the fact that the barrier width is much larger than the well widths for both wells, which predetermines only slight penetration of the wave functions into the neighboring well, and the effect of tunneling itself is only slightly supressed, as the barrier thickness is increased. At the same time, the activation energy is at least three time higher that the optical phonon energy, which cannot be explained on the basis of existing theory.     

Direct gap recombination in germanium at high excitation level and low temperature

In highly excited germanium and at low temperatures, a luminescence is observed at energies above the indirect gap at an energy of 0.880 eV. This luminescence is generally observed in connection with a broadened electron-hole-drop emission line thus indicating an increased density in the plasma state. At the same time, absorption in that energy range is reduced drastically. This luminescence is attributed to direct recombination processes between the Γ₂ conduction band minimum and the Γ₂₅ valence band maximum. The measurements we report concern the dependence of this luminescence on excitation conditions keeping the bath temperature constant at 2K. Line-shape, line-width and energetic position indicate that free carrier band-to-band recombination and no excitonic effects are involved. The dependence of the luminescence intensity on excitation power is well explained by a coupling by Auger processes between the indirect and the direct gap in germanium.     

Stoichiometry-dependent deep levels in undoped p-type Al0.38Ga0.62As grown by liquid phase epitaxy

Photocapacitance (PHCAP) and photoluminescence (PL) measurements were applied to unintentionally doped p-type Al_0.38Ga_0.62As grown by liquid phase epitaxy using the temperature difference method under controlled vapor pressure. PHCAP spectra revealed three dominant deep levels at E_v+0.9, E_v + 1.45, and E_v+1.96 eV, and a deep level at E_v+0.9−1.5 eV which was not neutralized by forward bias injection. These level densities increase with increasing arsenic vapor pressure and net shallow acceptor density. Furthermore, PL spectra reveal a deep level at 1.6–1.7 eV. The PL intensity of this deep level increases with increasing arsenic vapor pressure. These deep levels are thought to be associated with excess As.     

Photoconductivity and luminescence of CuInSe2 single crystals at a high level of optical excitation

The luminance-current and spectral characteristics of photoluminescence of the CuInSe₂ single crystals are studied. The superlinear portion of the excitation-intensity dependence of photoconductivity at low excitation intensities in compensated p-CuInSe₂ crystals is explained on the basis of a recombination model. The emission band that peaked at 0.98 eV in the n-CuInSe₂ photoluminescence spectrum corresponds to radiative recombination of electrons at the donor level with a depth of 0.04 eV. The maximum in the band intensity corresponds to the energy gap between the trap level and the valence band.     

Two sources of excitation of photoluminescence of porous silicon

The change occurring in the photoluminescence spectra and the photoluminescence excitation spectra during aging of porous-silicon samples in air and in vacuum has been investigated. It was found that the character of the photoluminescence changes occurring during aging depends on the wavelength of the exciting light: In the case of excitation in the visible-range band of the luminescence excitation spectrum (λ_exc>490 nm) the photoluminescence decreases and in the case of excitation in the ultraviolet band it predominantly increases. It is shown that the two bands of the luminescence excitation spectrum (visible and ultraviolet) correspond to two different objects on the surface of the porous layer. Fiz. Tekh. Poluprovodn. 31, 908–911 (August 1997)     

Effect of iodine impurity on relaxation of photoexcited silver chloride

The time and temperature dependences of relaxation of excited AgCl and AgCl:I crystals is studied by the method of photostimulated flash of luminescence. The presence of iodine impurity in silver chloride gives rise to hole recombination (luminescence) centers and hole traps in the band gap. It is shown that the main contribution to the decrease in the concentration of electrons localized at deep traps is made by the recombination of electrons with holes released thermally from shallow localization levels (iodine-related centers). Estimation of activation energy for the relaxation process showed that these energies for the AgCl and AgCl:I samples under study are the same within the experimental error and are equal to E _{rel 1} = 0.01 ± 0.0005 eV for the initial stage of relaxation and E _{rel 2} = 0.09 ± 0.005 eV for the final state. This fact indicates that the majority of hole traps involved in the relaxation process in AgCl are related to iodine impurity. In the course of thermal relaxation in AgCl, relocalization of nonequilibrium charge carriers from shallow levels to deep levels is observed. The depth of the corresponding trap is E _arl = 0.174 ± 0.03 eV.     

Optical study of resonant states in GaN x As1−x

To detect resonant states in the conduction band of GaN_xAs_1?x , it is suggested to study the spectra of intrinsic photoluminescence at room temperature or under high-level excitation, which fills high-energy states in the allowed bands with carriers excited by light. Energy levels within the conduction band of bulk GaN_xAs_1?x (x ≤ 0.015) layers were detected by the suggested method. These levels form a band of about 0.07 eV in half-width, with its density-of-states peak lying at ～1.4 and ～1.48 eV above the top of the valence band at temperatures 290 and 80 K, respectively. The position of this peak with respect to the valence band is virtually independent of the nitrogen content for all the studied range, 0.0005 ≤ x ≤ 0.015. It is assumed that these states are related to different clusters of nitrogen atoms.     

Changes in optical properties of CdS nanoclusters in langmuir-blodgett films on passivation in ammonia

The optical properties of CdS nanoclusters are studied for samples in which the nanoclusters are embedded in the Langmuir-Blodgett film matrix or the matrix is removed by annealing in vacuum or ammonia atmosphere. After annealing the samples in vacuum or ammonia, the bands of emission from the nanoclusters and from the surface states appear in the photoluminescence spectrum, with the peaks at 2.9 or 2.7 eV and at 1.9 or 2.1 eV, respectively. It is found that, after treating the samples with ammonia, the photoluminescence of the nanoclusters becomes more intense, whereas the photoluminescence corresponding to recombination via the levels of the surface states becomes less intense. It is established that the increase in the photoluminescence intensity for the nanoclusters and the difference between the temperature dependence of the photoluminescence peak position and the temperature dependence of the band gap of bulk CdS are due to passivation of surface states in the nanoclusters. The experimental data are interpreted in the model of recombination of nonequilibrium charge carriers in the CdS nanoclusters, taking into account the exchange of charge carriers between the nanocrystals and trapping centers at their surface and the recombination of charge carriers via the levels of the surface states.     

Electronic structural study of liquid phase epitaxy grown AlxGa1−xAs:Be using temperature dependent (≥ 10 K) laser excited photoluminescence

Crystals of Al_0.26Ga_0.74As doPed witn Be were grown by liquid phase epitaxy (LPE), and their laser excited photoluminescence (PL) spectra were accurately explored in the bandgap to deep level energy range. The temperature dependent (10 K ≤ T ≤ 70 K) and laser excitation power dependent PL data revealed that the 1.808 eV recombination is of D-A origin. From the thermal quenching and other experiments a 13.9 meV activation energy was found and is believed to be D_Si. From the data about the 1.808 eV recombination and the one at 1.841 eV, it was also determined that the activation energy is 38 meV for A_Be. No radiative deep level defects were found to be present in these LPE AlGaAs:Be crystals. The unintentionally introduced donor creates D-A recombinations competitive with CB-A recombinations and suggest the exercise of greater care in LPE growth of AlGaAs.     

Photoluminescence in CuGaSe2

The photoluminescence spectra of CuGaSe₂ have been investigated at 76 K in the range 0.65 < λ < 1.1μ. The crystals were grown by iodine transport, and had p-type conductivity. The photoexcitation was provided by the single-line outputs of a CW Ar⁺ laser, and the luminescence spectra were analyzed using a m grating monochromator.The luminescence observed consisted of a rather sharp structure near the energy gap, plus two broad peaks at considerably smaller energies. The broad peaks, centered around 1.52 and 1.30 eV respectively, are attributed to transitions involving deep localized states. The sharper structure near the energy gap is resolved into two peaks, one at 1.72 eV and another at 1.68 eV. This structure is explained by radiative recombination of free excitons (peak at 1.72 eV), and by recombination of free electrons with bound holes (peak at 1.68 eV).A thermal treatment done on the CuGaSe₂ crystals changed both their electrical properties and their luminescence spectra. These changes produced by sample treatment, plus measurements of the excitation power dependence and the polarization of the luminescent radiation, are in agreement with the interpretation presented.     

Optical properties of photochromatic sulfur-doped chlorosodalite

Synthetic.photochromic sulfo-chlorosodalite, 6(NaAlSiO₄) ·2 NaCl(S), has been thoroughly investigated by measurements of optical absorption, photo-luminescence and cathodoluminescence. Depending on the sulfur ion form and concentration, the doped sodalite exhibits either sensitive tene-brescence or photoluminescence with long wavelength UV excitation. The photo-induced color absorption peaks at 5260A at 300°K with absorption coefficient, Δα_max >200 cm⁻¹ . This is by far the highest photo-induced absorption observed for synthetic chlorosodalite. At 80°K, the peak position of the absorption does not show significant shift within instrumental accuracy. In photoluminescence, the emission spectra as well as the excitation spectra are studied at both 300 and 78°K. Four characteristic spectral bands (IR, blue, red, and a band with oscillation in wavelength) are observed. The oscillatory S₂ ^- ion emission band starting about 2.35 eV and extending to lower energy and the IR band peaked at 1.4 eV are most efficiently excited by 3660A (3.4 eV), whereas the blue luminescence peaked at 2.7 eV has an excitation threshold of 3.9 eV. The red band is often masked by the oscillatory band and can be observed by higher energy excitation. The red and blue bands are also observable in the cathodoluminescence measurements of the sulfur-doped samples but not the undoped samples. Correlating the absorption, luminescence, and excitation spectral results, a quantitative model is derived to interpret the nature and the role of sulfur ions in the photochromic chlorosodalite material.     

Photoelectrical and electrical properties of polycrystalline CdxHg1−x Te layers on GaAs substrates

Temperature dependences of electrical conductivity, concentration, and mobility of electrons, as well as photoconductivity spectra and conductivity-illumination characteristics of Cd_0.8Hg_0.2Te polycrystalline layers grown on GaAs substrates are studied. The features of charge transport and photoconductivity of Cd_xHg_1?x Te/CdTe/GaAs structures are discussed. It is established that a high photoconductivity at a temperature of 300 K and a jump in conductivity-illumination characteristics at high levels of excitation are caused by the influence of electrically active grain boundaries, which produce the potential barriers for the drift and recombination of charge carriers. It is shown within the framework of the semiconductor barrier model with a random potential relief pattern that, for high levels of excitation by the radiation pulses of ruby or neodymium lasers, the height of potential barriers at the grain boundaries lowers due to screening by nonequilibrium carriers.     

Intraband Cooling in All‐Inorganic and Hybrid Organic–Inorganic Perovskite Nanocrystals

Intraband relaxation in all‐inorganic cesium lead tribromide (CsPbBr₃) and hybrid organic–inorganic formamidinium lead tribromide (FAPbBr₃) nanocrystals is experimentally investigated for a range of particle sizes, excitation energies, sample temperatures, and excitation fluences. Hot carriers in CsPbBr₃ nanocrystals consistently exhibit slower cooling than FAPbBr₃ nanocrystals in the single electron–hole pair per nanocrystal regime. In both compositions, long‐lived hot carriers (>3 ps) are only observed at excitation densities corresponding to production of multiple electron–hole pairs per nanocrystal—and concomitant Auger recombination. These presented results are distinct from previous reports in bulk hybrid perovskite materials that convey persistent hot carriers at low excitation fluences. Time‐resolved photoluminescence confirms the rapid cooling of carriers in the low‐fluence (single electron–hole pair per nanocrystal) regime. Intraband relaxation processes, as a function of excitation energy, size, and temperature are broadly consistent with other nanocrystalline semiconductor materials.