Felix Nippert

Temperature-dependent recombination coefficients in InGaN light-emitting diodes: Hole localization, Auger processes, and the green gap

We obtain temperature-dependent recombination coefficients by measuring the quantum efficiency and differential carrier lifetimes in the state-of-the-art InGaN light-emitting diodes. This allows us to gain insight into the physical processes limiting the quantum efficiency of such devices. In the green spectral range, the efficiency deteriorates, which we assign to a combination of diminishing electron-hole wave function overlap and enhanced Auger processes, while a significant reduction in material quality with increased In content can be precluded. Here, we analyze and quantify the entire balance of all loss mechanisms and highlight the particular role of hole localization.

Determination of recombination coefficients in InGaN quantum-well light-emitting diodes by small-signal time-resolved photoluminescence

We suggest a novel technique for the evaluation of the recombination coefficients corresponding to Shockley–Read–Hall, radiative, and Auger recombination that occur in InGaN/GaN-based light-emitting diodes (LEDs). This technique combines the measurement of the LED efficiency as a function of LED drive current with a small-signal time-resolved photoluminescence measurement of the differential carrier life time (DLT). Using the relationships between the efficiency and DLT following from the empirical ABC-model, one can evaluate all three recombination coefficients. The suggested technique is applied to a number of single- and multiple-quantum well LEDs to gain a deeper insight into the mechanisms ultimately limiting their efficiency.

Recombination dynamics in InxGa1−xN quantum wells—Contribution of excited subband recombination to carrier leakage

The recombination dynamics of InxGa1−xN single quantum wells are investigated. By comparing the photoluminescence (PL) decay spectra with simulated emission spectra obtained by a Schrodinger-Poisson approach, we give evidence that recombination from higher subbands contributes the emission of the quantum well at high excitation densities. This recombination path appears as a shoulder on the high energy side of the spectrum at high charge carrier densities and exhibits decay in the range of ps. Due to the lower confinement of the excited subband states, a distinct proportion of the probability density function lies outside the quantum well, thus contributing to charge carrier loss. By estimating the current density in our time resolved PL experiments, we show that the onset of this loss mechanism occurs in the droop relevant regime above 20 A/cm2.

Green (In,Ga,Al)P-GaP light-emitting diodes grown on high-index GaAs surfaces

We report on green (550–560 nm) electroluminescence (EL) from (Al0.5Ga0.5)0.5In0.5P–(Al0.8Ga0.2)0.5In0.5P double p–i–n heterostructures with monolayer–scale tensile strained GaP insertions in the cladding layers and light–emitting diodes (LEDs) based thereupon. The structures are grown side–by–side on high–index and (100) GaAs substrates by molecular beam epitaxy. Cross–sectional transmission electron microscopy studies indicate that GaP insertions are flat, thus the GaP–barrier substrate orientation–dependent heights should match the predictions of the flat model. At moderate current densities (~500 A/cm 2 ) the EL intensity of the structures is comparable for all substrate orientations. Opposite to the (100)–grown strictures, the EL spectra of (211) and (311)–grown devices are shifted towards shorter wavelengths (~550 nm at room temperature). At high current densities (>1 kA/cm 2

Differential carrier lifetime in InGaN-based light-emitting diodes obtained by small-signal frequency-domain measurements

Recently, a novel method for evaluation of recombination coefficients corresponding to Shockley-Read-Hall, radiative, and Auger recombination channels has been proposed, which combines measurements of the light emitting diode (LED) external quantum efficiency under continuous wave operation with the determination of non-equilibrium carrier differential life time (DLT) by small-signal time-resolved photoluminescence [Nippert et al., Jpn. J. Appl. Phys., Part 1 55, 05FJ01 (2016)]. In this work, we suggest an alternative technique, small-signal frequency-domain lifetime measurements, which is implemented more easily and capable of operating in a wider range of LED operating currents. The DLTs measured by both techniques are shown to agree well with each other, but saturate at low currents, contrary to the trend predicted by the well-known ABC-model. We discuss possible reasons for this deviation, as well as advantages and limitations of the measurement techniques.

Unintentional indium incorporation into barriers of InGaN/GaN multiple quantum wells studied by photoreflectance and photoluminescence excitation spectroscopy

InxGa1–xN/GaN single and multi quantum well (MQW) structures with x ≈ 0.13 were investigated optically by photoreflectance, photoluminescence excitation spectroscopy, and luminescence. Clear evidence of unintentional indium incorporation into the nominal GaN barrier layers is found. The unintentional In content is found to be around 3%. Inhomogeneous distribution of In atoms occurs within the distinct quantum well (QW) layers, which is commonly described as statistical alloy fluctuation and leads to the characteristic S-shape temperature shift of emission energy. Furthermore, differences in emission energy between the first and the other QWs of a MQW stack are found experimentally. This effect is discussed with the help of model calculations and is assigned to differences in the confining potential due to unwanted indium incorporation for the upper QWs.

Polarization-induced confinement of continuous hole-states in highly pumped, industrial-grade, green InGaN quantum wells

We investigate industrial-grade InGaN/GaN quantum wells (QWs) emitting in the green spectral region under high, resonant pumping conditions. Consequently, an ubiquitous high energy luminescence is observed that we assign to a polarization field Confined Hole Continuum (CHC). Our finding is supported by a unique combination of experimental techniques, including transmission electron microscopy, (time-resolved) photoluminescence under various excitation conditions, and electroluminescence, which confirm an extended out-of-plane localization of the CHC-states. The larger width of this localization volume surpasses the QW thickness, yielding enhanced non-radiative losses due to point defects and interfaces, whereas the energetic proximity to the bulk valence band states promotes carrier leakage.

Electronic excitations stabilized by a degenerate electron gas in semiconductors

Excitons in semiconductors and insulators consist of fermionic subsystems, electrons and holes, whose attractive interaction facilitates bound quasiparticles with quasi-bosonic character. In the presence of a degenerate electron gas, such excitons dissociate due to free carrier screening. Despite their absence, we found pronounced emission traces in the below-band-edge region of bulk, germanium-doped GaN up to a temperature of 100 K, mimicking sharp spectral features at high free electron concentrations (3.4E19–8.9E19 cm−3). Our interpretation of the data suggests that a degenerate, three-dimensional electron gas stabilizes a novel class of quasiparticles, which we name collexons. These many-particle complexes are formed by exchange of electrons with the Fermi gas. The potential observation of collexons and their stabilization with rising doping concentration is enabled by high crystal quality due to the almost ideal substitution of host atoms with dopants.Systems which sustain quasiparticle complexes can exhibit unique optical features and unusual physical properties. Here the authors investigate highly doped bulk semiconductors and provide experimental evidence to suggest a new type of neutral, degenerate electron gas-stabilized quasiparticle which they term a collexon.

Growth and structure of In0.5Ga0.5Sb quantum dots on GaP(001)

Stranski-Krastanov (SK) growth of In0.5Ga0.5Sb quantum dots (QDs) on GaP(001) by metalorganic vapor phase epitaxy is demonstrated. A thin GaAs interlayer prior to QD deposition enables QD nucleation. The impact of a short Sb-flush before supplying InGaSb is investigated. QD growth gets partially suppressed for GaAs interlayer thicknesses below 6 monolayers. QD densities vary from 5 × 109 to 2 × 1011 cm−2 depending on material deposition and Sb-flush time. When In0.5Ga0.5Sb growth is carried out without Sb-flush, the QD density is generally decreased, and up to 60% larger QDs are obtained.

Carrier localization in InGaN-based light-emitting diodes (Conference Presentation)

We review recent advances in the understanding of the green gap phenomenon, the drastic reduction of quantum efficiency of c-plane InGaN/GaN light-emitting diodes (LEDs) towards the green spectral region. In particular, we have decoupled the contributions of Shockley-Read-Hall recombination, quantum-confined Stark effect and hole localization in the random alloy. We show that the latter, significantly increasing with Indium content, plays a crucial role in the reduction of efficiency, as localized holes do not only possess lower overlap with delocalized electrons in the quantum well, but also appear to enhance Auger recombination. For our study we use an electro-optical pump and probe scheme[1], which is most suitable to obtain differential carrier lifetimes in device operating conditions. In combination with conventional pulsed electroluminescence measurements, the internal quantum efficiency and recombination rates of the different processes can be determined. Temperature-dependent analyses then allow to assign recombination losses to the different underlying limitations (i.e. random alloying, polarity, defect density)[2]. [1] F. Nippert et al., Japanese Journal of Applied Physics 55, 05FJ01 (2016) [2] F. Nippert et al., Applied Physics Letters 109, 161103 (2016)