Bound Exciton Lines Research Articles

Luminescence from Bound Multiexcition Complexes (BMEC) in direct polar semiconductors

Localization energies and luminescence line positions for BMEC's in direct polar semiconductors are theoretically investigated. Our results for bound polaron and bound exciton exhibit excellent agreement with the best ones known from literature. In case of higher complexes significant stability is obtained for filled electron shells ( m=1,7,19), whereas gaps of instability are located between them. Luminescence lines are located between free exciton and bound exciton lines.

A photoluminescence study of beryllium-doped GaAs grown by molecular beam epitaxy

The 300-K photoluminescence internal quantum efficiency of beryllium-doped GaAs grown by molecular beam epitaxy at 580 °C from As4 under arsenic stable conditions is limited by nonradiative recombination with τNR = 3.5 to 5.6×10−10 s. τNR remains in this range while 1.3×1015 < NA−ND < 8×1017 cm−3. Layers grown at fixed NA−ND = 3×1016 cm−3 and constant thickness exhibited a 140-fold increase in internal efficiency as the growth temperature was raised from 480 to 660 °C. The corresponding internal efficiencies were calculated to be 0.054%, ⩾ 0.69%, and ⩾ 7.6%, respectively. The 6.5-K photoluminescence spectra of layers grown at 580, 620, and 660 °C showed no significant differences and exhibited none of the new bound exciton lines reported by Kunzel and Ploog to be present in molecular beam epitaxial GaAs grown from As4 below 610 °C.

Photoluminescence of carbon-implanted GaAs

The 5-K photoluminescence (PL) spectra of undoped semi-insulating GaAs are observed as a function of carbon implantation dose (after 850°C annealing using a SiO2 cap). Results show that although the intensity of the bound exciton line at 1.514 eV remains fairly independent of carbon concentration up to 1017 cm−3 carbon, the intensity of the conduction band to acceptor peak at ∼1.494 eV increases monotonically with carbon implant dose. This is the first direct confirmation that the 1.494-eV transition is due to carbon. The observed correlation between implanted carbon doses and PL intensities at high excitation levels is the basis of an analytic technique for C assessment in GaAs.

Acceptor bound exciton absorption and luminescence in germanium

Absorption and luminescence is reported, associated with the creation and decay of excitons bound to the acceptors gallium, indium and thallium in germanium. The principal bound exciton lines exhibit a triplet structure, which is a common feature of acceptor bound exciton spectra in other semiconductors. Particular attention is directed to prominent excited state structure which has not been clearly resolved in other materials.

Enhancement of long lifetime lines in photoluminescence from Si:In

We report a sample treatment technique which increases by a factor of between 50 and 10,000 the intensity of the long-lived P, Q and R lines previously observed in photoluminescence spectra of Si:In. The intensity of the indium bound exciton line is essentially unaffected. By studying these lines as a function of temperature, excitation density and delay time after pulsed excitation, we conclude that all three transitions are associated with the same isoelectronic binding center; contrary to prior assertions.

High-purity ZnSe grown by liquid phase epitaxy

A study was performed to establish the origin and nature of background compensating impurities in undoped ZnSe layers grown by liquid phase epitaxy on ZnSe substrate wafers in a low-contamination-level environment. The width of bound exciton lines in low-temperature photoluminescence spectra was used to define the quality of the material, and the energy of the lines was used to identify these low-level impurities. The sharpest spectra occurred in layers grown rapidly on a previously grown buffer layer indicating the importance of impurity outdiffusion from the substrate into the growing layer. The sharpness of these bound exciton lines indicates that the total concentration of electrically active impurities (NA+ND) is <1017/cc, an estimate which is confirmed by mass spectroscopy.

Concentration broadening of bound-exciton spectral lines

We report on an investigation of the broadening of bound exciton (BE) spectral lines at high impurity concentration for shallow acceptors in Si. The broadening is mainly due to the effect of overlapping of wave functions and is calculated by taking account of the dominant binary interaction. The calculated spectral shape for a nondegenerate component in the BE spectra is found to be asymmetric. We find the broadening is strongly dependent on the impurity concentration. The calculated full width at half maximum is about 1 meV for a concentration of 5 \ifmmode\times\else\texttimes\fi{} ${10}^{17}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ and is less than 0.01 meV for concentrations below 5 \ifmmode\times\else\texttimes\fi{} ${10}^{16}$ ${\mathrm{cm}}^{\ensuremath{-}3}$.

Shallow-acceptor, donor, free-exciton, and bound-exciton states in high-purity zinc telluride

Photoluminescence was measured at 1.6 K on ZnTe crystals, both nonintentionally doped high-purity and lightly P- or As-doped samples. Luminescence was excited mostly with a tunable dye laser close to the band gap. Ground states of the shallow-acceptor bound excitons are identified and attributed to the various acceptors by means of excitation spectra taken on two-hole satellites of these bound-exciton lines. Excited bound-exciton states are also obtained from the excitation spectra. The coupling of the electron and holes in the bound-exciton states appears to be very different for the various acceptors even for almost identical exciton localization energy. The excitation spectra of these two-hole luminescence satellites also exhibit negative spectral features attributed to the free-exciton $1S$, $2S$, and $3S$ levels. We derive the free-exciton binding energy ${E}_{\mathrm{FE}}(1S)=13.2\ifmmode\pm\else\textpm\fi{}0.3$ meV and the band-gap energy ${E}_{g}=2.3941\ifmmode\pm\else\textpm\fi{}0.0004$ eV. Excitation spectra were also taken on the donor-acceptor pair bands involving the Li, P, As, and the Cu acceptor. These excitation spectra yield $s$ and $p$ symmetric excited acceptor states and also excited donor levels. The latter yield 18.3 \ifmmode\pm\else\textpm\fi{} 0.3 meV binding energy of the predominant shallow donor. Higher excited $\mathrm{nS}$ states ($n\ensuremath{\ge}4$) for the Cu, Li, Ag, and the still unidentified $k$ acceptor are derived from two-hole series measured on unintentionally doped samples. Our results thus represent the most complete information available to date on excited shallow-acceptor states in ZnTe. The donor-acceptor pair excitation spectra contain features due to the creation of $\mathrm{TO}(\ensuremath{\Gamma})$ and $\mathrm{LO}(\ensuremath{\Gamma})$ phonons and also impurity-dependent lattice vibrations. Two-electron satellite lines are observed for resonant excitation at the shallow-donor bound exciton. The results agree with those from the excitation spectra taken on the various donor-acceptor pair bands.

Excited states of bound excitons in ZnO

The bound exciton lines I 5, I 6, and I 7 8 in ZnO at 5 K are studied in the energetic region of the free excitons and below by means of excitation spectroscopy. Apart from the maxima at the free exciton energies, additional resonances are observed and interpreted as the direct creation of bound excitons in excited states which lie 6meV and 4.5 meV above the ground states.

The puzzle of double-doped Si(B, In): Sharp line series in near-band-edge photoluminescence

We observe a large number of new lines in the near-band-edge (1060 to 1125 meV) photoluminescence of double-doped Si(B, In). The indium bound exciton lines appear as expected, however no bound exciton or bound multiexciton lines associated with boron are seen. The intensity of the new lines decreases relative to the intensity of the indium bound exciton line with increasing exciton power and decreasing temperature in the range from 1.2 to 20 K studied.

The isoelectronic trap iodine in AgBr

Iodine in AgBr behaves as an isoelectronic trap with the hole being bound first. The bound-exciton zero-phonon photoluminescence line is split by exchange into an A-line and a B-line separated by 0.1 meV. The Zeeman splitting at H=80 kG is isotropic in agreement with iodine being a substitutional point-like impurity in AgBr. In the low-temperature limit optic and acoustic phonons contribute to the phonon side wing with nearly the same coupling strength. At temperatures below 10K, up to n=11 phonons are observed in the phonon side band. The authors present experimental evidence that these are due to the emission of n optic phonons which, for n>3, are statistically independent. A multiphonon model which neglects the k dependence of transition rates is introduced; however, the model is useful only qualitatively.

Indium bound exciton luminescence in silicon

A high-resolution photoluminescence study is presented of the bound exciton line and its 4 meV low energy satellite in Si:In. The excitation and temperature dependence and the decay times suggest that the satellite line originates from the decay of an exciton bound to the indium acceptor. We tentatively correlate this line with excitations of the final state indium acceptors into a vibrational excited level.

The nature of the predominant acceptors in high quality zinc telluride

We report very sharp bound exciton luminescence spectra in high quality melt-grown very lightly compensated ZnTe, p-type with N A- N D in the low 10 +15 cm -3. Bound exciton localisation energies at seven shallow neutral acceptors with E A between ~55 and ~150 meV are very insensitive to E A. Optical absorption and dye laser luminescence excitation spectroscopy were necessary to obtain a full separation of the transitions due to different acceptors, together with a study of certain ‘two-hole’ luminescence satellites in which the acceptor is left in a series of orbital states after bound exciton decay. Two shallow acceptors are P Te and As Te, a third possibly Li Zn while a fourth, relatively prominent in our best undoped crystals, may be a complex. A deeper, 150 meV acceptor, frequently reported in the ZnTe literature and electrically dominant in most of our undoped crystals has the Zeeman character of a point defect. We present clear evidence from our spectra that this energy does not represent the binding of a single hole at a doubly ionized cation vacancy, a popular attribution since 1963. This acceptor may be covered by another impurity, possibly Cu Zn. We also report bound phonon effects, lifetime broadening of excited bound exciton states and observe a single unidentified donor with E D ~18.5 meV. This energy is determined using selective dye laser excitation at the weak neutral donor bound exciton line and from the onset of valence band to ionized donor photo-absorption.

Temperature Dependence of Raman Scattering and Exciton Luminescence Spectra in ZnTe

Secondary omission spectra have been studied in p -type ZnTe in the 4.2–170 K temperature range under the band-to-band excitation by an Ar + ion laser. The 1 L O , 2 L O and 3 L O Raman-like lines have been observed besides the free- and bound-exciton luminescence lines and their L O -phonon replicas. Temperature dependences of the line intensity, spectral position and width are compared among various radiation components. The observed temperature quenchings of the Raman-like lines are explained by the increase of the energy denominators in the Raman tensors due to the thermal shift of the exciton energy.

A high resolution investigation of the recombination radiation from Si containing the acceptors B, Al, Ga, In and Tl

A high resolution investigation has been made of the luminescence from Si doped with the acceptors B, Al, Ga, In and Tl in the spectral energy range 0.96–1.16 eV using 50 keV electron excitation. Luminescence from Tl-doped material has not previously been reported. Particular attention is directed to the splitting of the no-phonon and phonon-assisted bound exciton lines which has been detected for the first time in luminescence studies. Thermalizing doublet structure has been observed for Al, Ga and In-doped Si and a thermalizing triplet structure for tl-doped material, and three possible models for the fine structure are considered. Satellite lines on the low energy side of the bound exciton lines, which have previously only been observed in B, P and Li-doped Si and attributed to multiple bound exciton complexes, are reported for Al and Ga-doped material. Recent models for these lines are discussed together with their relationship with electron-hole droplet condensation. Structure associated with free electron to bound hole transitions are reported and discussed.

Fine structure in the bound exciton and multiple bound exciton luminescence from aluminium-doped silicon

A high-resolution investigation has been made of the luminescence associated with the decay of excitons bound to neutral Al acceptors in Si. The principal bound exciton line, associated with the transition A degrees X to A degrees , shows a thermalizing structure due to J-J coupling between the two holes and the electron at the acceptor site. The first satellite line on the low-energy side of the principal bound exciton line shows a doublet structure with a component separation and intensity ratio similar to two of the components of the principal line, identifying it with the transition A degrees XX to A degrees X. The second satellite line is a singlet. These observations support one of the recent models for multiple bound exciton transitions.

Fine Structure of the Luminescence from Excitons and Multiexciton Complexes Bound to Acceptors in Si

My high-resolution studies of the bound-exciton luminescence in Ga-doped Si reveal a triplet structure. Also, I report for the first time bound multiexciton complexes associated with Ga, which are similar to those previously reported for Si doped with P, Li, and and B. In the case of Ga, however, the $m=2$ bound-multiexciton complex luminescence is found to be split by an amount equal to the splitting between two of the bound-exciton lines, strongly suggesting that the $m=2$ complex decays into the bound exciton.

Pair spectra, edge emission, and the shallow acceptors in melt-grown ZnSe

The nature of edge emission bands in the photoluminescence spectra of as-grown, annealed, and electron-irradiated melt-grown ZnSe crystals at 4.2\ifmmode^\circ\else\textdegree\fi{}K has been investigated. Five different edge emission bands associated with donor-acceptor pair recombination are identified. These bands have their zero-phonon peaks at 4566, 4587, 4595, 4606, and 4625 \AA{}. A high-energy series, free-to-bound recombination band, is observed at 77\ifmmode^\circ\else\textdegree\fi{}K in association with the band at 4625 \AA{}. The bands at 4566, 4587, and 4595 \AA{} are seen only in annealed crystals; the band at 4625 \AA{} is seen only in as-grown crystals; whereas the band at 4606 \AA{} is observed in annealed and as-grown crystals. Discrete pair lines have been resolved only for the bands at 4566 and 4587 \AA{}. It is recognized that the band at 4606 \AA{} is the previously studied group-III-donor-${\mathrm{Li}}_{\mathrm{Zn}}$ recombination band and that the band at 4587 \AA{} is the as yet unidentified complex pair band reported by Dean and Merz. A study of the bound exciton lines occurring with the edge emission indicates that the band at 4625 \AA{} arises from group-III-donor-${\mathrm{Na}}_{\mathrm{Zn}}$ recombination. The acceptor ionization energy, ${E}_{A}$, of ${\mathrm{Na}}_{\mathrm{Zn}}$ is deduced to be 122 meV. From the analysis of the discrete pair lines associated with the bands at 4566 and 4587 \AA{} and from the results of electron irradiation and annealing experiments it is suggested that these two bands involve the same donor and acceptor and that the shorter-wavelength band arises from preferential pairing and the longer-wavelength band from random pairing of the donor and acceptor centers. The donor and the acceptor are tentatively assigned to be the doubly ionized zinc interstitial, ${\mathrm{Zn}}_{i}$, and the singly ionized complex (${\mathrm{Na}}_{\mathrm{Zn}}{V}_{\mathrm{Se}}$), respectively. The ionization energy of the complex center, (${\mathrm{Na}}_{\mathrm{Zn}}{V}_{\mathrm{Se}}$), is estimated from the corresponding bound exciton line to be 80-100 meV. The pair band at 4595 \AA{} is tentatively assigned to a group-III-donor-(${\mathrm{Li}}_{\mathrm{Zn}}{V}_{\mathrm{Se}}$) transition, and this acceptor has ${E}_{A}=110\ensuremath{-}130$ meV, estimated from the corresponding bound exciton line.

Indirect near edge emission in pure AgBr

AbstractThe emission of pure AgBr single crystals is studied in the vicinity of the indirect band gap from 1.5 to 50 K under various excitation conditions. The spectra are analysed with special respect to properties of the phonons involved in the indirect transitions, the lineshape of the free exciton emission which is found dependent on exciting frequency, and the dependence of some weakly bound exciton lines on temperature and excitation power. Results obtained for a narrow line close to the free exciton emission do not support the proposed model of a recombination of possibly Bose‐Einstein condensed free triplet excitons.

Low temperature photoluminescence in Ag-doped ZnSe

We have studied the low temperature photoluminescence of melt-grown, cubic, Ag-doped ZnSe single crystals. The emission lines of excitons bound to the interstitial Ag donor, the substitutional Ag acceptor on a zinc site, and two acceptor complexes involving Ag, have been identified. The binding energies of the Ag i donor and the Ag Zn acceptor are estimated from the corresponding bound exciton lines to be 33.7 and 240 meV respectively. The two acceptor complexes have binding energies 230 and 560 meV, estimated from the corresponding bound exciton lines. Luminescence of a neutral Ag complex has also been identified and it is found to arise in two ways: (a) from the recombination of a hole bound to the neutral complex with an electron trapped at a distant donor producing a pair spectrum, (b) from the decay of an exciton bound at the complex.