Above-gap Excitation Research Articles

Nonradiative centers in InAs quantum dots revealed by two-wavelength excited photoluminescence

We studied nonradiative recombination centers in MBE-grown InAs/GaAs quantum dot (QD) structures with photoluminescence (PL) peak energies between 1.12 and 1.29 eV by the scheme of two-wavelength excitation. Temporally chopped below-gap excitation (BGE) light of 0.75 eV was superposed on a CW above-gap excitation light of 1.59 eV on the sample surface, and the resultant PL intensity change due to the BGE (BGE effect) was measured. Observed 25–35% decrease in PL intensity at 80–90 K implies a first discrimination of a pair of nonradiative centers activated at 0.75 eV inside the band gap of the QD region. Due to trap-filling, the BGE effect showed saturation with increasing BGE power. Its temperature dependence below 50 K was different from that of quantum wells, reflecting carrier dynamics peculiar to the QD.

Emission spectra from AlN and GaN doped with rare earth elements

Luminescent properties of GaN and AlN based semiconductors containing rare earth metals of Gd and Dy are studied. Cathodoluminescent spectra from AlGdN show a clear and sharp peak at 318 nm following LO phonon satellites. Photoluminescence spectra from GaDyN by the above-gap excitation also show several peaks in addition to the broad luminescence band emission. For GaGdN, the sharp PL peaks are also observed at 650 and 670 nm, and they are assigned to the intra-f orbital transitions by their time decay measurements. The broad band at around 365 nm for AlGdN, 505 nm for GaGdN and GaDyN are commonly observed. The origin of these broad bands is discussed.

Excitation-Wavelength Dependent and Time-Resolved Photoluminescence Studies of Europium Doped GaN Grown by Interrupted Growth Epitaxy (IGE)

AbstractThe emission properties of Eu doped GaN thin films prepared by interrupted growth epitaxy (IGE) were investigated through excitation-wavelength dependent and time-resolved photoluminescence (PL) studies. Under above-gap excitation (333-363 nm) large differences were observed in the Eu3+ PL intensity and spectral features as a function of Ga shutter cycling time. The overall strongest red Eu3+ PL intensity was obtained from a sample grown with a Gashutter cycling time of 20 minutes. The main Eu3+ emission line originating from 5D0→ 7F2 transition was composed of two peaks located at 620 nm and 622 nm, which varied in relative intensity depending on the growth conditions. The room-temperature emission lifetimes of the samples were non-exponential and varied from ∼50 νs to ∼200 νs (1/e lifetimes). Under resonant excitation at 471 nm (7F0→5D2) all samples exhibited nearly identical PL spectra independent of Ga shutter cycling time. Moreover, the Eu3+ PL intensities and lifetimes varied significantly less compared to above-gap excitation. The excitation wavelengths dependent PL results indicate the existence of different Eu3+ centers in GaN: Eu, which can be controlled by the Ga shutter cycling time.

Effect of modulation-doping on luminescence properties of plasma assisted MBE-grown GaN/AlGaN quantum well

In order to improve the crystal quality of GaN-based light emitting devices, photoluminescence (PL) characterization of below-gap states in plasma assisted MBE-grown GaN/AlGaN quantum well (QW) structures has been done by utilizing a below-gap excitation (BGE) light in addition to an above-gap excitation light. The decrease of the band-edge PL intensity due to the addition of the BGE of 1.17 eV indicates the presence of an energy-matched below-gap state in the two-wavelength excited PL. In continuation to our previous efficiency improvement by applying modulation-doping to GaAs/AlGaAs QW's, we focused on several undoped and Si-doped GRN/AlGaN QW's. Experimental results showed that Si modulation-doping reduces the density of below-gap states in the QW region, hence it is promising for increasing internal quantum efficiency of GaN-based QW's.

Photoluminescence studies of rare earth (Er, Eu, Tm) in situ doped GaN

The emission properties of rare earth (RE)-doped GaN are of significant current interest for applications in full color displays, white lighting technology, and optical communications. We are currently investigating the photoluminescence (PL) properties of RE (Er, Eu, Tm)-doped GaN thin-films prepared by solid-source molecular beam epitaxy. The most intense visible PL under above-gap excitation is observed from GaN:Eu (red: 622 nm) followed by GaN:Er (green: 537 nm, 558 nm), and then GaN:Tm (blue: 479 nm). In this paper, we present spectroscopic results on the Ga-flux dependence of the Er 3+ PL properties from GaN:Er and we report on the identification of different Eu 3+ centers in GaN:Eu through high-resolution PL excitation (PLE) studies. In addition, we observed an enhancement of the blue Tm 3+ PL from AlGaN:Tm compared to GaN:Tm. Intense blue PL from Tm 3+ ions was also obtained from AlN:Tm under below-gap pumping.

Spectral and time-resolved photoluminescence studies of Eu-doped GaN

We report on spectral and time-resolved photoluminescence (PL) studies performed on Eu-doped GaN prepared by solid-source molecular-beam epitaxy. Using above-gap excitation, the integrated PL intensity of the main Eu3+ line at 622.3 nm (5D0→7F2 transition) decreased by nearly 90% between 14 K and room temperature. Using below-gap excitation, the integrated intensity of this line decreased by only ∼50% for the same temperature range. In addition, the Eu3+ PL spectrum and decay dynamics changed significantly compared to above-gap excitation. These results suggest the existence of different Eu3+ centers with distinct optical properties. Photoluminescence excitation measurements revealed resonant intra-4f absorption lines of Eu3+ ions, as well as a broad excitation band centered at ∼400 nm. This broad excitation band overlaps higher lying intra-4f Eu3+ energy levels, providing an efficient pathway for carrier-mediated excitation of Eu3+ ions in GaN.

Thermal quenching of photoluminescence from Er-doped GaN thin films

The green (537 and 558 nm) and near infrared (1.54 μm) photoluminescence (PL) spectra of Er-doped GaN thin films have been investigated as a function of temperature, excitation wavelength, and pump intensity. Thermal quenching measurements showed that the integrated green Er3+ PL intensity (4S3/2/2H11/2→4I15/2) remained nearly constant up to 150 K, but decreased at higher temperatures due to a less efficient Er3+ excitation. The integrated infrared Er3+ PL intensity (4I13/2→4I15/2) was found to be temperature-independent up to 250 K, but decreased slightly at higher temperatures due to the onset of non-radiative decay. Pump intensity PL studies revealed that the above-gap excitation cross-section is more than two orders of magnitude greater than the below-gap excitation cross-section. Within a simplified three-level model, the above-gap excitation cross-section was estimated to be ∼10−16 cm2. This result indicates that Er3+ ions can be excited efficiently through carrier-mediated processes in a forward-biased GaN:Er light emitting device.

Below-gap recombination dynamics in GaN revealed by time-resolved and two-wavelength excited photoluminescence

In continuation to the steady state detection of below-gap states by combining a below-gap excitation (BGE) light with an above-gap excitation (AGE) light, we have studied the time-response of both donor-acceptor pair luminescence (DAP) and yellow luminescence (YL) in undoped GaN after switching off the BGE light. An extremely slow recovery of YL with a time constant of up to 28 s was observed, which is interpreted as a relaxation process of electrons from a below-gap state chosen by a BGE energy of 1.17 eV. Trap parameters were quantitatively determined by studying the intensity change and subsequent recovery time of DAP as a function of AGE and BGE power densities. The BGE energy dependence of the intensity change and subsequent recovery time of DAP and YL implies an energy distribution of traps in the band-gap of GaN. The combination of time-resolved PL measurement under two-wavelength excitation scheme was shown to be powerful for understanding the carrier recombination dynamics including YL among below-gap states.

Extremely Slow Relaxation Process of a Yellow-Luminescence-Related State in GaN Revealed by Two-Wavelength Excited Photoluminescence

In addition to detection of nonradiative recombination centers by a below-gap excitation (BGE) technique at steady state, we have studied the time-response of both donor-acceptor pair luminescence and yellow luminescence (YL) in GaN after switching off the BGE light. An extremely slow recovery of YL with a time constant of up to 28 s was observed, which is interpreted as a relaxation process of electrons from the state chosen by a BGE energy of 1.17 eV. The dependence of recovery time on the BGE energy and on the above-gap and below-gap excitation power densities became clear for the first time; it reflects the carrier recombination dynamics including YL among below-gap states.

Effects of material growth technique and Mg doping on Er3+ photoluminescence in Er-implanted GaN

Photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopies have been carried out at 6 K on the ∼1540 nm I13/24–I15/24 emissions of Er3+ in Er-implanted and annealed GaN. These studies revealed the existence of multiple Er3+ centers and associated PL spectra in Er-implanted GaN films grown by metalorganic chemical vapor deposition, hydride vapor phase epitaxy, and molecular beam epitaxy. The results demonstrate that the multiple Er3+PL centers and below-gap defect-related absorption bands by which they are selectively excited are universal features of Er-implanted GaN grown by different techniques. It is suggested that implantation-induced defects common to all the GaN samples are responsible for the Er site distortions that give rise to the distinctive, selectively excited Er3+PL spectra. The investigations of selectively excited Er3+PL and PLE spectra have also been extended to Er-implanted samples of Mg-doped GaN grown by various techniques. In each of these samples, the so-called violet-pumped Er3+PL band and its associated broad violet PLE band are significantly enhanced relative to the PL and PLE of the other selectively excited Er3+PL centers. More importantly, the violet-pumped Er3+PL spectrum dominates the above-gap excited Er3+PL spectrum of Er-implanted Mg-doped GaN, whereas it was unobservable under above-gap excitation in Er-implanted undoped GaN. These results confirm the hypothesis that appropriate codopants can increase the efficiency of trap-mediated above-gap excitation of Er3+ emission in Er-implanted GaN.

Near infrared (1.54 μm) luminescence properties of erbium doped gallium nitride

The photoluminescence (PL) properties of in-situ Er doped GaN prepared by metalorganic molecular beam epitaxy (MOMBE) have been investigated. The GaN:Er films were grown on sapphire or silicon substrates. The oxygen and carbon background concentrations in the films were measured to be in the order of ∼10 20 and ∼10 21 cm −3, respectively. Both types of GaN:Er samples showed intense 1.54 μm PL at room temperature under below-gap optical excitation. For above-gap excitation, a greatly reduced Er 3+ luminescence intensity was observed. Pump intensity dependent PL studies revealed that the Er 3+ excitation efficiency under above-gap excitation is roughly a factor of ∼30 smaller compared to below-gap excitation. Based on the efficient Er 3+ below-gap excitation process a hybrid GaN:Er/sapphire/InGaN LED was developed.

Selective enhancement of 1540 nm Er3+ emission centers in Er-implanted GaN by Mg codoping

The ∼1540 nm I13/24 to I15/24 Er3+ photoluminescence (PL) and photoluminescence excitation (PLE) spectra of Er-implanted Mg-doped GaN reveal a selective enhancement of one of the nine different Er3+ centers observed previously in PL and PLE studies of Er-implanted undoped GaN. These Er3+ PL spectra are excited selectively by pump wavelengths that correspond to broadband, below-gap absorption bands associated with different Er3+ centers. In the Er-implanted, Mg-doped GaN, both the 1540 nm PL spectrum characteristic of the so-called violet-pumped Er3+ center and the ∼2.8–3.4 eV (violet) PLE band that enables its selective excitation are significantly enhanced by Mg doping. In addition, the violet-pumped PL center dominates the above-gap-excited Er3+ PL spectrum of Er-implanted Mg-doped GaN, whereas it was nearly unobserveable under above-gap excitation in Er-implanted undoped GaN. These results confirm our hypothesis that appropriate codopants can increase the efficiency of trap-mediated above-gap excitation of Er3+ emission in Er-implanted GaN.

Comparison of the Optical Properties of Er3+ Doped Gallium Nitride Prepared by Metalorganic Molecular Beam Epitaxy (Mombe) and Solid Source Molecular Beam Epitaxy (SSMBE)

We report on the luminescence properties of Er doped GaN grown prepared by metalorganic molecular beam epitaxy (MOMBE) and solid-source molecular beam epitaxy (SSMBE) on Si substrates. Both types of samples emitted characteristic 1.54 µm PL resulting from the intra-4f Er3+ transition 4I13/2→4I15/2. Under below-gap excitation the samples exhibited very similar 1.54 µm PL intensities. On the contrary, under above-gap excitation GaN: Er (SSMBE) showed ∼80 times more intense 1.54 µm PL than GaN: Er (MOMBE). In addition, GaN: Er (SSMBE) also emitted intense green luminescence at 537 nm and 558 nm, which was not observed from GaN: Er (MOMBE). The average lifetime of the green PL was determined to be 10.8 µs at 15 K and 5.5 µs at room temperature. A preliminary lifetime analysis suggests that the decrease in lifetime is mainly due to the strong thermalization between the 2H11/2 and 4S3/2 excited states. Nonradiative decay processes are expected to only weakly affect the green luminescence.

Optical Nonlinearities in the Band Edge Region of Highly Excited (In)GaN Thin Films Studied via Femtosecond and Nanosecond Optical Pump-Probe Spectroscopy

We present the results of a detailed study of the band edge absorption changes in GaN and InGaN thin films induced by the presence of excess photo-generated free carriers. The 1s A and B free excitonic resonances in GaN are shown to decrease with increasing above-gap nanosecond optical excitation due to screening by free carriers and exciton–exciton scattering. The decrease in excitonic absorption with increasing above-gap excitation is accompanied by a significant increase in the below-gap absorption coefficient. To further explore this behavior, femtosecond nondegenerate optical pump–probe experiments were also performed using an amplified Ti : sapphire laser. Exciton saturation due to screening by free carriers and excitonic phase space filling was again observed, along with a modest amount of below-gap induced absorption attributed to band gap renormalization. Similar experiments were performed on InGaN thin films. The band edge absorption changes observed in InGaN films were found to be significantly different than those observed in GaN.

Intrinsic Luminescence from Li2O Crystals Excited in the Exciton-Band Region

Photoluminescence of lithium oxide (Li2O) has been studied under the excitation with UV light in the fundamental absorption region at low temperatures. Two bands appear at 3.70 and 4.75 eV. The 3.70 and 4.75 eV bands are stimulated on the low- and high-energy sides of the exciton reflection band peaking at 7.02 eV, respectively. An interesting fact is that there appears no luminescence under the above-gap excitation. The 4.75 eV band is ascribed to radiative decay of self-trapped excitons, and the 3.70 eV band is related to unavoidably existing defects or impurities. Both a broadening of the line width and a quenching of the intensity of the 4.75 eV band are also investigated in the temperature range 9 to 300 K. The present result is discussed in comparison with the theoretical model of the self-trapped exciton in this material.

Comparison of the Optical Properties of Er3+ Doped Gallium Nitride Prepared by Metalorganic Molecular Beam Epitaxy (MOMBE) and Solid Source Molecular Beam Epitaxy (SSMBE)

AbstractWe report on the luminescence properties of Er doped GaN grown prepared by metalorganic molecular beam epitaxy (MOMBE) and solid-source molecular beam epitaxy (SSMBE) on Si substrates. Both types of samples emitted characteristic 1.54 µm PL resulting from the intra-4f Er3+ transition 4I13/2→4I15/2. Under below-gap excitation the samples exhibited very similar 1.54 µm PL intensities. On the contrary, under above-gap excitation GaN: Er (SSMBE) showed ∼80 times more intense 1.54 µm PL than GaN: Er (MOMBE). In addition, GaN: Er (SSMBE) also emitted intense green luminescence at 537 nm and 558 nm, which was not observed from GaN: Er (MOMBE). The average lifetime of the green PL was determined to be 10.8 µs at 15 K and 5.5 µs at room temperature. A preliminary lifetime analysis suggests that the decrease in lifetime is mainly due to the strong thermalization between the 2H11/2 and 4S3/2 excited states. Nonradiative decay processes are expected to only weakly affect the green luminescence.

Optical Characterization of Erbium Doped III-Nitrides Prepared by Metalorganic Molecular Beam Epitaxy

We are currently engaged in a systematic study of the optical properties of Er doped III-nitrides prepared by metalorganic molecular beam epitaxy (MOMBE). Under below-gap excitation it was observed that GaN: Er samples with [O]∼1020 cm−3 and [C]∼1021 cm−3 luminesce at 1540 nm with an intensity of more than two orders of magnitude greater than samples with low oxygen and carbon concentrations (< 1019 cm−3). Associated with the different oxygen and carbon concentrations were different thermal quenching behaviors and below-gap absorption bands. Interestingly, for above-gap excitation only small differences in absolute Er3+ PL intensity and quenching behavior were observed for samples of varying O and C content. Initial lifetime studies were performed and showed a rather unusual short decay time of ∼100 μs at room temperature. In order to gain more insight in the Er3+ PL, a comparison of the integrated PL intensity and lifetime was performed for the temperature range 15-500K. The result reveals that the Er3+ PL quenches above room temperature due to the onset of non-radiative decay and the reduction in excitation efficiency. All samples were also investigated for visible luminescence. Red luminescence was observed from GaN: Er on sapphire substrates under below-gap excitation.

Distribution of below-gap states in undoped GaAs/AlGaAs quantum wells revealed by two-wavelength excited photoluminescence

The spatial distribution of below-gap states in undoped GaAs/AlGaAs quantum-well structures and their effect on the luminescence intensity has been determined by a spectroscopic measurement of two-wavelength excited photoluminescence. Tuning the photon energy of above-gap excitation revealed that these below-gap states were formed neither in the GaAs well layers nor at the GaAs/AlGaAs hetero-interface, but in the AlGaAs barrier layers. The properties of below-gap states in AlGaAs were found to be strongly dependent on growth temperature.

Below-Gap Spectroscopy of Undoped GaAs/AlGaAs Quantum Wells by Two-Wavelength Excited Photoluminescence

By using an improved method of two-wavelength excited photoluminescence, we observed a considerable decrease in the band-to-band photoluminescence intensity of an undoped GaAs/AlGaAs quantum well structure when a below-gap excitation was superposed on an above-gap excitation. This intensity quenching was attributed to the enhanced nonradiative recombination through below-gap states which were activated by the below-gap excitation. By changing the energy of above-gap excitation, it was shown that these below-gap states were formed neither inside GaAs well layers nor at the GaAs/AlGaAs heterointerface, but inside AlGaAs barrier layers. Tuning the photon energy of below-gap excitation revealed that the activation energy of the nonradiative recombination process via these below-gap states is around 1.2 eV. Lowering the above-gap excitation density to a single-photon-counting region enabled us to characterize below-gap states spectroscopically without the need of a high-power tunable laser source for the below-gap excitation.

Excitation mechanisms of multiple Er3+ sites in Er-implanted GaN

Site-selective photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopies carried out at 6K on the ∼1540 nm 4I13/2→4I15/2 emissions of Er3+ in Er-implanted GaN have revealed the existence of four different Er3+ sites and associated PL spectra in this semiconductor. Three of these four sites are excited by below-gap, impurity- or defect-related absorption bands, with subsequent nonradiative energy transfer to the Er3+ 4f electrons; a fourth site is excited by direct Er3+ 4f shell absorption. PLE spectra obtained by selectively detecting Er3+ PL from each of the three sites pumped by broad below-gap absorption bands are compared with the PLE spectra of broad PL bands attributed to implantation damage-induced defects in the Er-implanted GaN. This comparison enables us to distinguish broad-band, below-gap optical excitation processes for Er3+ emission that are attributable to (1) absorption due to implantation damage-induced defects; (2) absorption due to defects or impurities characteristic of the as-grown GaN film; and (3) an Er-specific absorption band just below the band gap which may involve the formation of an Er-related isoelectronic trap. The two sites excited by impurity-or defect-related absorption bands are also strongly pumped by above-gap excitation, while the sites pumped by the Er-related trap and direct 4f shell absorption are not. This observation indicates that excitation of Er3+ luminescence in crystalline semiconductor hosts by either optical or electrical injection of electron-hole pairs is dominated by trap-mediated carrier capture and energy transfer processes. These trap-mediated processes may also control the thermal quenching of Er3+ emission in semiconductors.