T. Malinauskas

Determination of free carrier bipolar diffusion coefficient and surface recombination velocity of undoped GaN epilayers

Time-resolved nondegenerate four-wave mixing experiments were performed on 2.6-μm-thick GaN epilayers grown by metalorganic chemical-vapor deposition using picosecond pulses at 355 and 1064 nm for grating recording and probing, respectively. Measurements of free carrier grating kinetics at various grating periods Λ in range from 3 to 10 μm allowed determination of nonequilibrium carrier bipolar diffusion coefficient D=1.7 cm2/s at 300 K. Substitution of the D value into two-dimensional carrier transport model allowed fitting of the whole set of grating kinetics at various Λ varying surface recombination velocity S and linear recombination time τR. This procedure provided us a value of S=5×104 cm/s as well as carrier lifetime of ∼1 ns.

Optical monitoring of nonequilibrium carrier lifetime in freestanding GaN by time-resolved four-wave mixing and photoluminescence techniques

Optical monitoring of nonequilibrium carrier dynamics was performed in freestanding GaN. Four-wave mixing kinetics directly provided carrier lifetime of 5.4ns in the layer, while complementary measurements by photoluminescence technique revealed the fast transients with subnanosecond decay time. Numerical modeling of photoluminescence decay taking into account the carrier spatial-temporal dynamics allowed us to attribute an origin of the fast photoluminescence transients to carrier diffusion to the bulk and to reabsorption of the backward emission. The studies demonstrated carrier diffusion limited applicability of the time-resolved photoluminescence technique for carrier lifetime measurements in a high quality thick III-nitride layers.

Implementation of diffractive optical element in four-wave mixing scheme for ex situ characterization of hydride vapor phase epitaxy-grown GaN layers.

A holographic beam splitter has been integrated into a picosecond four-wave mixing (FWM) scheme. This modification significantly simplified the procedure of dynamic grating recording, thus making the FWM technique an easy-to-use tool for the holographic characterization of wide band gap materials. The novel FWM scheme was applied for characterization of hydride vapor phase epitaxy-grown undoped GaN layers of different thickness. It allowed the determination of carrier lifetime, diffusion coefficient, and carrier diffusion length by optical means, as well as the study of carrier recombination peculiarities with respect to dislocation and excess carrier density.

The determination of high-density carrier plasma parameters in epitaxial layers, semi-insulating and heavily doped crystals of 4H-SiC by a picosecond four-wave mixing technique

We applied a picosecond four-wave mixing technique for measurements of carrier lifetimes and diffusion coefficients in highly excited epitaxial layers, semi-insulating and heavily doped 4H-SiC substrates. Optical carrier injection at two different wavelengths (266 and 355 nm) allowed us to vary the depth of the excited region from 1–2 µm to 50 µm, and thus determine photoelectric parameters of carrier plasma in the density range from 2 × 1016 to 1019 cm−3. Strong dependence of carrier lifetime and mobility on carrier density was found in the epitaxial layers. The origin of fast decay components, not resolved previously by photoluminescence and free-carrier absorption techniques in SiC, was attributed to nonlinear carrier recombination. Numerical modelling provided a value of bimolecular recombination coefficient equal to B = (2–4) × 10−11 cm3 s−1 and verified a surface recombination velocity S = 4 × 104 cm s−1. In heavily doped crystals, nonlinear carrier recombination reduced the carrier lifetime down to 1.1 ns, while in semi-insulating ones a lifetime of 1.5–2.5 ns was measured. Temperature dependences of four-wave mixing provided monopolar carrier mobility in heavily doped and bipolar one in semi-insulating crystals, and revealed the contribution of ionized impurity and phonon scattering mechanisms.

Dynamics of free carrier absorption in InN layers

Carrier dynamics in highly excited InN epitaxial layers was investigated in the 1550–2440 nm (0.8–0.51 eV) spectral range by using a femtosecond differential transmission technique. A transition from induced bleaching to induced absorption was observed for probing energy of 90 meV below the bandgap of the samples. The decay of the induced free carrier absorption provided the averaged lifetime of the total nonequilibrium carriers. In the carrier density range of Δn=1018–1020 cm−3, the density-dependent recombination mechanism was attributed to trap-assisted Auger recombination with decay rate 1/τ=BTAARΔn, with BTAAR in the range (4–30)×10−10 cm3 s−1 for layers with different defect densities.

Impact of carrier localization, recombination, and diffusivity on excited state dynamics in InGaN/GaN quantum wells

We apply a number of all-optical time-resolved techniques to study the dynamics of free carriers in InGaN quantum structures under high excitation regime. We demonstrate that carrier lifetime and diffusion coefficient both exhibit a substantial dependence on excitation energy fluence: with increasing carrier density, carrier lifetime drops and diffusivity increases; these effects become more apparent in the samples with higher indium content. We discuss these experimental facts within a model of diffusion-enhanced recombination, which is the result of strong carrier localization in InGaN. The latter model suggests that the rate of non-radiative recombination increases with excitation, which can explain the droop effect in InGaN. We use the ABC rate equation model to fit light induced transient grating (LITG) kinetics and show that that linear carrier lifetime drops with excitation (i.e. excess carrier density). We do not observe any influence of Auger recombination term, CN3, up to the maximum carrier density that is limited due to the onset of very fast stimulated recombination process. To support these conclusions, we present spectrally resolved differential transmission data revealing different recombination rates of carriers in localized and extended states.

Optical and electron beam studies of carrier transport in quasibulk GaN

Variable temperature light-induced transient grating technique combined with electron beam-induced current measurements in situ in a scanning electron microscope were employed for carrier transport studies in quasibulk hydride-vapor phase epitaxy grown undoped GaN layers. Diffusion length of carriers independently determined from both techniques was found to increase with temperature in the range from 70 to 400 K. This increase was attributed to the temperature-induced growth of carrier lifetime, as was confirmed by light-induced transient grating measurements below 300 K and by cathodoluminescence above room temperature.

Growth of InN and In-Rich InGaN Layers on GaN Templates by Pulsed Metalorganic Chemical Vapor Deposition

InN and In-rich InGaN layers have been grown on GaN templates using the pulsed metalorganic chemical vapor deposition (MOCVD) technique and compared with analogous layers grown by conventional MOCVD. Structural investigations were performed using x-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Surface morphology was studied using atomic force microscopy. Electrical and optical properties were studied using Hall measurements and photoluminescence (PL) spectroscopy. All layers were free of metal droplets. InN grown using pulsed MOCVD showed fairly low background electron concentration (ne = 8 × 1018 cm−3 to 9 × 1018 cm−3) and high electron mobility (μe = 644 cm2 V−1 s−1). Structural studies revealed increase of size (from 100 nm to 500 nm) and decrease of density of InN islands at higher growth temperatures. For In-rich InGaN layers (In content 68% and 80%) the density of islands was similar to that in InN, while the diameter varied from 50 nm to 150 nm. Inhomogeneities of In and Ga distribution in the layers resulting in broadened XRD lines and PL bands are discussed.

Optical Characterization of Defect-Related Carrier Recombination and Transport Features in GaN Substrates and CVD Diamonds

Defect related carrier recombination and transport properties have been investigated in differently doped HVPE GaN substrates and CVD diamond layers. Carrier generation by interband transitions or by deep-defect photoexcitation were realized for studies of GaN samples by using picosecond pulses at 351 nm or 527 nm. This allowed to create favorable conditions for radiative and nonradiative recombination in the crystals and reveal peculiarities of photoelectrical properties of high and low density plasma in undoped, doped, and compensated GaN. In CVD diamonds, carrier diffusion length was found equal to ~ 0.5 μm and non-dependent on nitrogen density, while the carrier lifetime varied from 0.2 to 0.6 ns.

A systematic study of light extraction efficiency enhancement depended on sapphire flipside surface patterning by femtosecond laser

We report on the investigation of light extraction efficiency enhancement at the flip-side surface of sapphire patterned by using femtosecond laser pulse irradiation. The enhancement was measured by means of fluorescence and confocal microscopies. Theoretical simulation (FDTD) gave the results. The variation of patterning geometry provided by different gap and pit size, by triangle and square array configuration revealed compactness of the pit network as an optimum condition for the enhancement. Maximum enhancement over 40% was obtained for the triangle array configuration. The appearance of ripples at the bottom of the pit corresponded to the enhancement. A deeper insight about consequences to radiative effects influenced by the ripples is presented.