Electroluminescence Spectroscopy Research Articles

Low temperature electroluminescence spectroscopy of high electron mobility transistors on InP

Electroluminescence spectroscopy of short gate high-electron-mobility transistors (HEMTs) on InP substrates is performed at cryogenic temperatures. Electroluminescence is a reliable tool to investigate impact ionization as compared to studies based on gate current which depend on the weakness of the intrinsic gate current intensity. In on-state biased devices, a low energy (0.7–0.9 eV) recombination band is observed which is related to radiative recombination of carriers created by impact ionization in the low band gap InGaAs channel. The evolution of the luminescence intensity versus bias applied to the device shows that the electroluminescence intensity and impact ionization depend on two competing parameters: the electric field in the gate–drain access area and the drain current intensity. We show that the so-called ‘‘kink’’ effect, which is a noticeable increase of the output conductance and which is observed at relatively moderate drain bias (600–750 mV) in our devices, is not correlated with impact ionization. The electroluminescence of the device in the off state is also investigated. This study allows the direct observation of impact ionization in the off-biased channel of InP-based HEMTs. In this low current regime, the electroluminescence intensity follows the electric field, i.e., the drain–gate voltage until breakdown of the device occurs. The voltage breakdown of the device in the off state is discussed in terms of impact ionization in the InGaAs channel due to hot carriers originating from the gate leakage current. Finally, a method for reducing or avoiding impact ionization in these devices, i.e., for increasing the device reliability, is discussed.

Time-resolved electroluminescence spectroscopy of resonant tunneling in GaAs-AlAs superlattices

The carrier transport through a resonantly coupled GaAs-AlAs superlattice is investigated by time-resolved electroluminescence. Ultrashort current pulses are used to electrically inject electrons and holes into an externally biased superlattice. The time evolution of the emitted light is recorded, which allows us to directly monitor the temporal development of the carrier transport and the associated dynamics of the electric field. At resonance a strong reduction of the initial rise time of the electroluminescence signal is observed.

Elastic strain and enhanced light emission in dry etched Si/Si1-xGex quantum dots

Quantum dots of 50 ~ 60 nm diameter fabricated from both Si/Si1-xGex (x = 0.1 ~ 0.3) strained layer superlattices and a strain symmetried Si9/Ge6 superlattice were investigated by a combination of Raman scattering, photoluminescence, and electroluminescence spectroscopy. It was found that, in addition to an enhanced luminescence intensity of the dots by over two orders of magnitude and improved luminescence quenching temperature, all of the nanostructure dots have residual built-in elastic strains, which are of the order of ~50% of the values in corresponding pseudomorphic heterostructures. This result suggests a possible mechanism for explaining the huge enhancement of the optical efficiency in our luminescence measurements.

Γ-X Mixing studied by electroluminescence spectroscopy under hydrostatic pressure in a GaAs/AlAs p-i-n superlattice resonant tunnelling device

Electroluminescence measurements under hydrostatic pressure have been performed to study the Γ-X mixing in GaAs AlAs p-i-n double barrier resonant tunnelling structures incorporating superlattice energy filters in the emitter and collector regions. For a sample with a 5 nm quantum well, an anti-crossing with the lowest confined X-states in the AlAs barriers is observed under hydrostatic pressure. The observed anti-crossing agrees well with the expected position and pressure coefficient of the confined X-states. A sample with a narrower 3.1 nm quantum well exhibits a recombination which is higher in energy than the lowest confined X-states in the AlAs barrier and decreases with an anomalously large pressure coefficient.

Electroluminescence spectroscopy of AlGaAs/InGaAs and AlGaAs/GaAs high-electron-mobility transistors

Electroluminescence of pseudomorphic InGaAs high-electron-mobility transistors (HEMTs) on GaAs and of standard AlGaAs/GaAs HEMTs with ultrashort gate lengths is investigated at room and cryogenic temperatures. The spectral distribution of the emitted radiation is analyzed in the 0.7–1.6 eV energy range. In the case of pseudomorphic transistors, three different recombination lines originating from distinct layers are observed: a broad luminescence band around 0.8–0.9 eV; a doublet arising from confined subbands in the InGaAs layer; and a single peak coming from the GaAs substrate. The energy position and the temperature dependence of these different lines under various biases give valuable information on the physical mechanisms which occur under high-electric-field nonstationary transport: lattice self-heating (≊150 K at high-bias conditions), origin of the impact ionization in the channel of the quantum-well layer, and perpendicular transfer of hot holes which recombine with the barrier DX centers thus leading to the broad luminescence band. These characteristics are supported by comparison with analog standard HEMTs and with electrical measurements.

Electroluminescence spectroscopy in a high magnetic field of the ballistic-electron energy distribution in single-barrier heterostructures.

A high-resolution electroluminescence study of the hot-electron energy distribution, and of the nature of the tunneling processes, in GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As single-barrier p-i-n diodes is reported. Application of quantizing magnetic fields permits electron injection from narrow, well-defined electron states into the collector region, even at high emitter density. As a result ballistic-electron peaks, accompanied by their LO-phonon cascade, are clearly resolved. Cross-barrier recombination is also observed. The energy distribution of the ballistic electrons is shown to reflect exactly that of the emitter electrons, even though the density is \ensuremath{\sim}${10}^{5}$ times lower, thus demonstrating that the two-dimensional to three-dimensional tunneling process is elastic and independent of the in-plane motion. The energy relaxation of the ballistic electrons is shown to be dominated by LO-phonon emission, with energy randomization by, e.g., carrier-carrier scattering playing no significant role.

Carrier Recombination in A-SI:H P-I-N Devices Studied by PL and EL Spectroscopies

ABSTRACTBoth photoluminescence (PL) and electroluminescence (EL) energy spectra were studied in the temperature range of 80 to 300 K on a-Si:H p-i-n structures. The EL spectrum depends on several parameters such as the i-layer thickness and the sample structure with or without a buffer layer (b-layer), while the PL spectrum shows no difference when those parameters are varied. Comparing PL and EL in 0.5 μm p-i-n devices with and without a buffer layer, we found that (a) at 80 K, the main-band peak energy is 1.3 eV for PL and 1.2 eV for EL; (b) the PL spectral line shape does not change with the insertion of a buffer layer, but the EL spectra show more enhanced main-band luminescence with the buffer layer; (c) the temperature dependence of the PL intensity shows a slope of 26 K which is similar to that of a-Si:H films, but the EL efficiency shows a weaker temperature dependence that varies with the diode structure.

Observation of ballistic transport in double-barrier resonant-tunneling structures by electroluminescence spectroscopy.

We report a direct observation by electroluminescence (EL) spectroscopy of ballistic-electron transport in double-barrier resonant-tunneling GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As p-i-n diodes. The samples studied contain two confined electron states (e1 and e2) and consequently two resonances in the current versus bias characteristic. When biased for electron tunneling through e2, an analysis of EL intensities permits a quantitative determination of the ratio (1:16 and 1:203 for the two samples studied) of the ballistic current flowing directly through e2 to the current flowing sequentially through e1.

Magneto-optical studies of ballistic electron transport in single barrier heterostructures

We report an investigation of ballistic electron transport in GaAs/AlGaAs p-i-n single barrier structures with magnetic fields of up to 14T applied parallel to the tunneling direction (B//z). The energy distribution and relaxation processes of the non-equilibrium electron population injected into the p-doped collector from the Landau levels of the emitter accumulation layer are studied by means of electroluminescence (EL) spectroscopy. The observation of emitter Landau level structure in the ballistic electron EL spectra shows that the 2D to 3D tunneling process is elastic. In addition to the ballistic electron EL, cross-barrier recombination between the electron and hole accumulation layers is observed. This allows a precise determination of the initial energy distribution of the injected electrons.

Electroluminescence spectroscopy of resonant tunnelling in GaAs-AlAs superlattices

Resonant tunnelling between different subbands of adjacent wells is investigated with electroluminescence spectroscopy as a function of forward bias voltage. The observed near-bandgap electroluminescence lines indicate a strong inhomogeneity in the electric field as well as in the carrier distribution. This interpretation is supported by calculations. Electroluminescence lines originating from excited conduction and valence subband states demonstrate an efficient occupation of these subbands by resonant tunnelling.

Optical spectroscopy of inverted electron populations in double-barrier resonant-tunnelling structures

We present results of photoluminescence and electroluminescence spectroscopy carried out on GaAs/ Al 0.33Ga 0.67As double-barrier resonant-tunnelling structures having quantum wells of either 200 or 250 Å width. At high bias, luminescence recombination is observed from electrons in highly excited ( n > 2) quantum-well (QW) states of the structures. Analysis of the luminescence intensities shows that when the structures are biased at the fourth electron resonance, a population inversion occurs between the n = 4 and n = 3 confined electron levels. In addition, when the 250 Å QW structure is biased at the fifth resonance, a population inversion occurs between the n = 5 and n = 4 confined levels. We show that such population inversions are explained by a rate-equation analysis of the excited-state populations.

Electroluminescence spectroscopy of resonantly coupled GaAs-AlAs superlattices

The resonant coupling of different electronic subbands between adjacent wells in GaAs-AlAs superlattices is investigated with electroluminescence (EL) spectroscopy as a function of the applied forward bias. The EL efficiency, i.e., the EL intensity normalized to the current, exhibits a resonant behavior. An efficient occupation of the second subband by sequential resonant tunneling is observed at higher bias.

Hole and electron traps in AlGaAs single and double heterojunction red light-emitting diodes

ABSTRACT Single and double heterostructure AlGaAs red LEDs, p+–N and n+–P AlGaAs-GaAs heterodiodes have been fabricated by liquid phase epitaxy (LPE). The presence and characteristics of deep centres located near the injection-active boundaries has been studied. Capacitance-temperature, electroluminescence, photocurrent and deep-level transient spectroscopy (DLTS) measurement have been performed. In n+–P structures a new Zn-related deep hole majority trap, at about 0·3 eV above the valence band, was found. In all p+–N heterojunctions, in addition to a majority electron trap related to the Te-dopant (DX defects), some acceptor-like traps have been detected.

Electron transport in Heliobacterium chlorum wholecells studied by electroluminescence and absorbance difference spectroscopy

An electric field pulse induced luminescence signal was observed in suspensions of whole cells of the photosynthetic green bacterium Heliobacterium chlorum . This signal is associated with electron transport in the reaction center. The dependence of the electroluminescence on the delay between the flashand the pulse and the absorbance kinetics at 800 and 553 nm revealed the following properties of electron transport in whole cells: (1) most of P-798 + was rereduced by cytochrome c -553 in two phases with half-times of 0.11 and 0.7 ms and equalamplitudes (this is considerably faster than the timesreported for membrane fragments); (2) if reduced cytochrome c -553 is not available, P-798 + reacts back with a reduced acceptor in 20 ms; (3) oxidized cytochrome c -553 is rereduced in about 20 ms, presumably by an external donor, because it does not take place after resuspension of the cells in Tris buffer. In membrane fragments, spheroplasts and cells stored at −20°C, cytochrome oxidation is inhibited and no electroluminescence can be observed.

Damage constant and deep-level transient spectroscopy in neutron irradiated GaAsP alloys

Damage produced in VPE GaAs1− x P x . alloys by fast neutron irradiation at room temperature was studied, in light emitting diodes, through the evolution of device carrier lifetime, photoluminescence, electroluminescence and transient capacitance spectroscopy characteristics. Neutron fluxes were in the 1013−1014 neutrons/cm2 range so as not to heavily damage the devices. Damage constants are 10−5 to 10−6 cm2/s for 0.3 ≤x ≤ 1. The carrier removal rate was ≈; 10 cm−1. Deep-level transient spectroscopy in n-type layers revealed that fast neutrons created a broad center atE c − 0.7 eV, and at a ≈;1 cm−1 generation rate. For thex ≥ 0.4 composition range studied, trap characteristics and introduction rates were rather independent ofx. From photocapacitance quenching measurements it is suggested that the neutron generated centers are EL2-related.