V. G. Mansurov

Electrical properties and deep traps spectra of N-polar and Ga-polar AlGaN films grown by molecular beam epitaxy in a wide composition range

The polarity type, surface morphology, electrical properties, and deep trap spectra were studied for undoped AlxGa1−xN films (x=0–0.6) grown by molecular beam epitaxy on on-axis (0001) sapphire using composite buffers consisting of low temperature (LT) AlN nucleation layer, AlN, and AlN/AlGaN superlattice. It is shown that the films grow with N-polarity if the LT AlN layer is deposited under N-rich conditions and with Ga-polarity for the LT AlN layers deposited under Al-rich conditions. For both polarities the film morphology was acceptable for fabrication of typical GaN-based devices. It is demonstrated that the Ga-polar AlGaN films are heavily compensated p-type, with the dominant acceptors believed to be due to C. N-polar films are n type, with the residual donors pinning the Fermi level being most likely due to Si. N-polar films show a high concentration of deep electron traps.

Critical phenomena in the β-(2×4) → α-(2×4) reconstruction transition on the (001) GaAs surface

The critical exponents of the β-(2×4) → α-(2×4) reconstruction phase transition on the (001) GaAs surface are determined experimentally. It is found that the phase transition is analogous to a van der Waals transition. The critical parameters Tc, Pc, and Θc have been measured experimentally. The mean field theory is applied, and three-parameter isotherms are obtained that agree with the experimental results at the following values of the parameters: Est = 0.36 eV, ΔE = 0.18 eV, and Ei = 0.134 eV. Precision measurements of the critical exponents β and δ are carried out. Their values β = 1/8 and δ = 15 indicate that the phase transition is truly two-dimensional.

Nonradiative recombination in GaN quantum dots formed in the AlN matrix

The mechanisms of temperature quenching of steady-state photoluminescence are studied for structures with hexagonal GaN quantum dots embedded in the AlN matrix. The structures are grown by molecular beam epitaxy. The study is conducted for structures with differently sized quantum dots, for which the peak of the photoluminescence band is in the range from 2.5 to 4.0 eV. It is found that the activation energy of thermal quenching of photoluminescence varies from 27 to 110 meV, as the quantum-dot height is decreased from 5 to 2 nm. A model is suggested to interpret the results. According to the model, the photo-luminescence signal is quenched because of the transfer of charge carriers from energy levels in the quantum dots to defect levels in the matrix.

Role of lateral interaction in the homoepitaxy of GaAs on the (001)-β(2 × 4) surface

The homoepitaxy of GaAs on the (001)-β(2 × 4) surface during molecular beam epitaxy is considered as a twodimensional first-order phase transition from the lattice gas of adsorbed growth components to the two-dimensional crystalline phase. In the context of the mean field theory of phase transitions, the parameters of lateral interaction between filled cells of the lattice gas are determined. The causes for the completion of the growth of a particular monolayer (self-ordering) before the beginning of growth of a new monolayer are clarified. The oscillations observed in the reflection high-energy electron diffraction experiments support the conclusions of the suggested theory of the phase transition for homoepitaxy.

Asymmetric c(4×4) → γ(2×4) reconstruction phase transition on the (001)GaAs surface

The c(4×4) → γ(2×4) reconstruction phase transition on the (001)GaAs surface is studied experimentally. It is shown that it is a first-order phase transition. The phase transition is found to exhibit a highly asymmetric hysteresis. The difference between the direct and inverse runs of the hysteresis is explained in terms of the mean field theory of an adsorption-induced phase transition by the substantial contribution of lateral multiparticle interactions in the adsorbate.

Photoluminescence kinetics slowdown in an ensemble of GaN/AlN quantum dots upon tunneling interaction with defects

The carrier recombination dynamics in an ensemble of GaN/AlN quantum dots is studied. The model proposed for describing this dynamics takes into account the transition of carriers between quantum dots and defects in a matrix. Comparison of the experimental and calculated photoluminescence decay curves shows that the interaction between quantum dots and defects slows down photoluminescence decay in the ensemble of GaN/AlN quantum dots.

MBE-grown AlGaN/GaN heterostructures for UV photodetectors

The MBE synthesis of AlGaN/GaN semiconductor heterostructures intended for UV photodetectors is considered. A technique for growing AlGaN layers and multilayered heterostructures is developed. It includes the nitridization of the sapphire substrate surface, the formation of a seed layer, and the growth of a buffer layer and undoped and doped AlGaN layers of different composition. The influence of growth conditions on the surface morphology, density of threading dislocations and other structural defects, and electrophysical and optical properties of individual AlGaN layers and respective heterostructures for UV photodetectors is investigated. The mathematical simulation of p-i-n photodiodes is made, and a process route for fabrication of AlGaN heterostructures is developed. Test AlGaN p-i-n photodiodes are prepared, and their performance is investigated.

2D AlN layer formation on (111)Si surface by ammonia MBE

An (0001)AlN ultrathin layer formation in ammonia MBE conditions have been investigated by RHEED. Two steps procedure was used: at first, well ordered (8×8) silicon nitride was prepared on the atomically clean Si surface under ammonia flux. Then AlN was formed by deposition of Al atoms onto the (8×8) surface. Temperature dependence of the AlN formation rate was obtained and desorption energy (Edes=2.1 eV) of Al atoms from the surface was estimated. For the first time the (4×4) superstructure of AlN on the (111)Si surface was observed by RHEED. Precise measurements of the AlN in-plain lattice parameter evolution during the initial stages of the AlN formation were carried out. The lattice constant of 3.08 A is found in good agreement with ab initio calculations for a graphene-like AlN.

Growth of AlGaN/GaN heterostructures with a two-dimensional electron gas on AlN/Al2O3 substrates

The possibility of using AlN/Al2O3 substrates to grow AlGaN/GaN hetero-epitaxial structures with a two-dimensional electron gas is studied. A method of calibrating the temperature of the substrates by measuring the thermal radiation spectrum is proposed. Differences between AlN/Al2O3 substrates that lead to differences in the electrophysical parameters of the grown structures are determined. AlN/Al2O3 substrates were used to grow AlGaN/GaN samples with a two-dimensional electron gas mobility in excess of 1300 cm2/(V · s) at an electron concentration in the channel higher than 1013 cm−2.

Origin of Below Band‐Gap Photoluminescence from GaN Quantum Dots in AlN Matrix

Stationary and time‐resolved photoluminescence of GaN quantum dots grown by MBE in an AlN matrix have been studied. Temperature dependence of photoluminescence peak related to the quantum dots evidences in intrinsic nature of photoluminescence. A huge red shift of the photoluminescence peak in time‐resolved photoluminescence spectra has been attributed to redistribution of carriers between small and large quantum dots.