E. E. Zavarin

Low-temperature method of cleaning p-GaN(0001) surfaces for photoemitters with effective negative electron affinity

The changes in the chemical composition, atomic structure, and electronic properties of the p-GaN(0001) surface upon chemical treatment in an HCl-isopropanol solution and vacuum annealing are investigated by x-ray photoelectron spectroscopy, high-resolution electron energy-loss spectroscopy, and low-energy electron diffraction. It is demonstrated that a considerable part of the surface gallium oxide is removed upon chemical treatment of the GaN surface. Subsequent annealing of the surface under vacuum at temperatures of 400–450°C leads to a decrease in the residual carbon and oxygen contamination to 3–5% of the monolayer. The preparation of a clean p-GaN(0001) surface with a (1×1) structure identical to that of the bulk unit cells is confirmed by the low-energy electron diffraction data. The cesium adsorption on the clean p-GaN surface results in a decrease in the work function by ∼2.5 eV and the appearance of an effective negative electron affinity on the surface. The quantum efficiency of the GaN photocathode at a wavelength of 250 nm is equal to 26%.

Effect of strain relaxation on active-region formation in InGaN/(Al)GaN heterostructures for green LEDs

Processes of active-region formation for green LEDs on the basis of multilayer strained InGaN/GaN nanoheterostructures have been studied. It is shown that the formation of structures of this kind is highly affected by elastic stress relaxation leading to a larger amount of indium incorporated into InGaN layers. For structures emitting in the blue spectral range, an increase in the number of quantum wells (QWs) from 1 to 10 does not lead to stress relaxation or to a shift of the emission wavelength, whereas for structures emitting in the green spectral range, raising the number of QWs from one to five causes a monotonic increase in the emission wavelength.

Analysis of the local indium composition in ultrathin InGaN layers

Experimental photoluminescence (PL) spectra and structural properties of the ultrathin InGaN insertions in an AlGaN matrix grown on a sapphire substrate were investigated. The PL emission mechanism was shown to be governed by the quantum dot (QD)-like indium-rich areas with size about 3 nm, which was determined from high resolution transmission electron microscopy image analysis. The local indium composition in QDs of the samples was estimated as about 26% using the suggested model of PL transition energy, which accounts for quantum confinement energy, spontaneous and piezoelectric fields.

Influence of the carrier gas composition on metalorganic vapor phase epitaxy of gallium nitride

The influence of hydrogen and nitrogen as carrier gases on the rates of gallium nitride (GaN) growth and etching in the process of metalorganic vapor phase epitaxy (MOVPE) have been studied. Based on these data, the possible roles of hydrogen and nitrogen in the events on the surface of an epitaxial GaN layer are considered.

Growth of AlGaN epitaxial layers and AlGaN/GaN superlattices by metal-organic chemical vapor deposition

Special features of metal-organic chemical vapor deposition of AlGaN epitaxial layers and AlGaN/GaN superlattices either in an Epiquip VP-50 RP research and development reactor (for a single wafer 2 in. in diameter) or in an AIX2000HT production-scale reactor (for up to six wafers 2 in. in diameter) are stud-ied. It is found that the dependence of the aluminum content in the solid phase on the trimethylaluminum (TMA) flux in a reactor levels off; this effect hinders the growth of the layers with a high aluminum content in both types of reactors and is more pronounced in the larger reactor (AIX2000HT). Presumably, this effect is a consequence of spurious reactions in the vapor phase and depends on the partial pressure of TMA in the reactor. The aluminum content in the layers can be increased not only by reducing the total pressure in the reactor but also by increasing the total gas flow through the reactor and reducing the trimethylgallium flux. The approaches described above were used to grow layers with a mole fraction of AlN as large as 20% in the AIX2000HT production-scale reactor at a pressure of 400 mbar (this fraction was as large as 40% at 200 mbar). AlGaN layers with the entire range of composition were grown in the Epiquip VP-50 RP reactor.

Nonequilibrium population of charge carriers in structures with InGaN deep quantum dots

Electronic and optical properties of ensembles of quantum dots with various energies of activation from the ground-state level to the continuous-spectrum region were studied theoretically and experimentally with the InGaN quantum dots as an example. It is shown that, depending on the activation energy, both the quasi-equilibrium statistic of charge carriers at the levels of quantum dots and nonequilibrium statistic at room temperature are possible. In the latter case, the position of the maximum in the emission spectrum is governed by the value of the demarcation transition: the quantum dots with the transition energy higher than this value feature the quasi-equilibrium population of charge carriers, while the quantum dots with the transition energy lower than the demarcation-transition energy feature the nonequilibrium population. A model based on kinetic equations was used in the theoretical analysis. The key parameters determining the statistic are the parameters of thermal ejection of charge carriers; these parameters depend exponentially on the activation energy. It is shown experimentally that the use of stimulated phase decomposition makes it possible to appreciably increase the activation energy. In this case, the thermal-activation time is found to be much longer than the recombination time for an electron-hole pair, which suppresses the redistribution of charge carriers between the quantum dots and gives rise to the nonequilibrium population. The effect of nonequilibrium population on the luminescent properties of the structures with quantum dots is studied in detail.

MOCVD-grown AlGaN/GaN heterostructures with high electron mobility

Specific features of MOCVD growth of AlGaN/GaN heterostructures have been studied. In the structures obtained, the 2D electron gas in the channel had a density of 1.2×1013 cm−2 and a mobility of 1290 cm2/(V s) at room temperature. The effect of the purity of starting components on the properties of the structure is studied.

Energy characteristics of excitons in structures based on InGaN alloys

Samples containing ultrathin InGaN layers that emit radiation in the spectral range from the ultraviolet to yellow region are studied. The samples are grown by metal-organic vapor-phase epitaxy. The Urbach energy, the localization energy of excitons, and the activation energy of charge carriers are determined to characterize radiative and nonradiative processes in the quantum dots and barriers of the structures. It is shown that these energy parameters are linearly dependent on the photon energy in the range from 3.05 to 2.12 eV. It is established that temperature variations in the emission intensity are due to the increase in the number of charge carriers thermally activated from the quantum wells into barriers as well as due to the enhancement of scattering of free excitons at defects.

Photoluminescence of localized excitons in InGan quantum dots

Photoluminescence spectra of samples with ultrathin InGaN layers embedded in AlGaN and GaN matrices are studied experimentally in the temperature range of 80 to 300 K. It is shown that the temperature dependences can be understood in the context of Eliseev’s model and that, in the active region of the structures under study, the dispersion σ of the exciton-localization energy depends on the average In content in InGaN-alloy layers. Furthermore, the Urbach energy EU, which characterizes the localization energy of excitons in the tails of the density of states, was determined from an analysis of the shape of the low-energy slope of the spectrum. It is shown that σ and EU, quantities representing the scale of the exciton-localization effects, vary linearly with the photoluminescence-peak wavelength in the range from the ultraviolet to the green region of the spectrum.

Thermal-Field Forward Current in GaN-Based Surface-Barrier Structures

The voltage and temperature dependences of the capacitance and forward current in surface-barrier Ni-n-GaN structures are experimentally studied. The results are compared with the Padovani-Stratton thermofield emission theory. It is established that, in a temperature range of 250–410 K, the forward current of the Ni-n-GaN surface-barrier structures (the electron density in GaN is ∼1017 cm−3) is caused by a thermofield emission of electrons, whose energy is ∼0.1 eV below the potential-barrier top.