S. Ruvimov

Radiative recombination in type‐II GaSb/GaAs quantum dots

Strained GaSb quantum dots having a staggered band lineup (type II) are formed in a GaAs matrix using molecular beam epitaxy. The dots are growing in a self‐organized way on a GaAs(100) surface upon deposition of 1.2 nm GaSb followed by a GaAs cap layer. Plan‐view transmission electron microscopy studies reveal well developed rectangular‐shaped GaSb islands with a lateral extension of ∼20 nm. Intense photoluminescence (PL) is observed at an energy lower than the GaSb wetting layer luminescence. This line is attributed to radiative recombination of 0D holes located in the GaSb dots and electrons located in the surrounding regions. The GaSb quantum dot PL dominates the spectrum up to high excitation densities and up to room temperature.

Microstructure of Ti/Al and Ti/Al/Ni/Au Ohmic contacts for n-GaN

Transmission electron microscopy has been applied to characterize the structure of Ti/Al and Ti/Al/Ni/Au Ohmic contacts on n‐type GaN (∼1017 cm−3) epitaxial layers. The metals were deposited either by conventional electron‐beam or thermal evaporation techniques, and then thermally annealed at 900 °C for 30 s in a N2 atmosphere. Before metal deposition, the GaN surface was treated by reactive ion etching. A thin polycrystalline cubic TiN layer epitaxially matched to the (0001) GaN surface was detected at the interface with the GaN substrate. This layer was studied in detail by electron diffraction and high resolution electron microscopy. The orientation relationship between the cubic TiN and the GaN was found to be: {111}TiN//{00.1}GaN, [110]TiN//[11.0]GaN, [112]TiN//[10.0]GaN. The formation of this cubic TiN layer results in an excess of N vacancies in the GaN close to the interface which is considered to be the reason for the low resistance of the contact.

Effect of Si doping on the dislocation structure of GaN grown on the A‐face of sapphire

Transmission electron microscopy, x‐ray diffraction, low‐temperature photoluminescence, and Raman spectroscopy were applied to study stress relaxation and the dislocation structure in a Si‐doped GaN layer in comparison with an undoped layer grown under the same conditions by metalorganic vapor phase epitaxy on (11.0) Al2O3. Doping of the GaN by Si to a concentration of 3×1018 cm−3 was found to improve the layer quality. It decreases dislocation density from 5×109 (undoped layer) to 7×108 cm−2 and changes the dislocation arrangement toward a more random distribution. Both samples were shown to be under biaxial compressive stress which was slightly higher in the undoped layer. The stress results in a blue shift of the emission energy and E2 phonon peaks in the photoluminescence and Raman spectra. Thermal stress was partly relaxed by bending of threading dislocations into the basal plane. This leads to the formation of a three‐dimensional dislocation network and a strain gradient along the c axis of the layer.

InAs–GaAs Quantum Pyramid Lasers: In Situ Growth, Radiative Lifetimes and Polarization Properties

We have realized injection lasers based on InAs–GaAs and InGaAs–GaAs quantum pyramids (QPs) with a lateral size ranging from 80 to 140 A. The structures with relatively small dots (~80 A) exhibit properties predicted earlier for quantum dot (QD) lasers such as low threshold current densities (below 100 Acm-2) and ultrahigh characteristic temperatures (T0=350–425 K). For operation temperatures above 100–130 K, T0 decreases and the threshold current density increases (up to 0.95–3.3 kAcm-2 at room temperature) due to carrier evaporation from QPs. Larger InAs QPs (~140 A) providing better carrier localization exhibit saturation of the ground-state emission and enhanced nonradiative recombination rate at high excitation densities. The radiative lifetime shows a weak dependence on the dot size in the range 80–140 A being close to ~1.8–2 ns, respectively. A significant decrease in radiative lifetime is realized in vertically coupled quantum dots formed by a QP shape-transformation effect. The final arrangement corresponds to a three-dimensional tetragonal array of InAs islands inserted in a GaAs matrix each composed of several vertically merging InAs parts. We achieved injection lasing in such an array for the first time.

Microstructure of Ti/Al ohmic contacts for n-AlGaN

Transmission electron microscopy was employed to evaluate the microstructure of Al/Ti ohmic contacts to AlGaN/GaN heterostructure field-effect transistor structures. Contact resistance was found to depend on the structure and composition of the metal and AlGaN layers, and on atomic structure of the interface. A 15–25-nm-thick interfacial AlTi2N layer was observed at the contact-AlGaN interface. Formation of such nitrogen-containing layers appears to be essential for ohmic behavior on n-type III-nitride materials suggesting a tunneling contact mechanism. Contact resistivity was found to increase with Al fraction in the AlGaN layer.

Self-organization processes in MBE-grown quantum dot structures

InAs quantum dots in a GaAs matrix have been prepared by molecular beam epitaxy using a self-organizing mechanism. A narrow size distribution of single dots of pyramidal shape (typically with a base of 12 ± 1 nm and a height of 4–6 nm) is created as directly imaged with plan-view and cross-section transmission electron microscopy. The dots exhibit self-organized short range order and preferentially align in rows along 〈100〉. The photoluminescence of the dot ensemble has, due to fluctuations in dot size, shape and strain, a FWHM of typically 50–60 meV. However, using highly spatially and spectrally resolved cathodoluminescence it is possible to directly excite a tiny fraction of all dots (typically only 30 dots). Under these excitation conditions the spectrum changes drastically into a series of ultrasharp lines with a FWHM < 0.15 meV, each originating from a different single InAs quantum dot. This directly visualizes their δ function-like density of electronic states, especially since the lines remain sharp even for kBT⪢FWHM.

Effect of matrix on InAs self-organized quantum dots on InP substrate

InAs self-organized quantum dots in In0.53Ga0.47As and In0.52Al0.48As matrices have been grown on InP substrates by molecular beam epitaxy. The dot size in InGaAs has been found to be 3–4 times larger, but the areal density about an order of magnitude smaller than that in InAlAs. Low-temperature photoluminescence (PL) of the InAs/InGaAs quantum dots is characterized by a narrow (35 meV) PL line as compared to that of InAs/InAlAs quantum dots (170 meV). Quantum dot formation increases the carrier localization energy as compared to quantum well structures with the same InAs thickness in a similar manner for both InAs/InGaAs and InAs/InAlAs structures. The effect of the barrier band gap on the optical transition energy is qualitatively the same for quantum well and quantum dot structures. The results demonstrate a possibility of controlling the quantum dot emission wavelength by varying the matrix composition.

Structural characterization of bulk GaN crystals grown under high hydrostatic pressure

This paper describes TEM characterization of bulk GaN crystals grown at 1500–1800Kin the form of plates from a solution of atomic nitrogen in liquid gallium under high nitrogen pressure (up to 20 kbars). The x-ray rocking curves for these crystals were in the range of 20–30 arc-sec. The plate thickness along thec axis was about 100 times smaller than the nonpolar growth directions. A substantial difference in material quality was observed on the opposite sides of the plates normal to thec direction. On one side the surface was atomically flat, while on the other side the surface was rough, with pyramidal features up to 100 nm high. The polarity of the crystals was determined using convergent-beam electron diffraction. The results showed that, regarding the long bond between Ga and N along the c-axis, Ga atoms were found to be closer to the flat side of the crystal, while N atoms were found to be closer to the rough side. Near the rough side, within 1/10 to 1/4 of the plate thickness, there was a high density of planar defects (stacking faults and dislocation loops decorated by Ga/void precipitates). A model explaining the defect formation is proposed.

Formation of coherent superdots using metal‐organic chemical vapor deposition

We demonstrate direct growth of electronically coupled zero‐dimensional structures forming a super‐quantum dot using metal‐organic chemical vapor deposition. After the first sheet with InGaAs pyramids is formed on GaAs surface, alternate short‐period GaAs‐InGaAs deposition leads to spontaneous formation of layered structures driven by the energetics of Stranski–Krastanow growth. As a result columnlike InGaAs structures each having a characteristic lateral size of ∼23 nm at the top and composed of many closely packed InGaAs parts are formed. The full width at half maximum of superdot luminescence of 28 meV at 8 K indicates good average uniformity of the superdot ensemble. Absorption is found to be resonant with luminescence.

Chemical and structural transformation of sapphire (Al2O3) surface by plasma source nitridation

The chemical, morphological, and structural characteristics of nitrogen plasma treated c-plane sapphire substrate surfaces were studied by x-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and transmission electron microscopy (TEM). The plasma treatment was carried out by exposing sapphire substrates (Ts=700 °C) to a flow of 35 sccm of activated nitrogen species generated by a constricted-plasma source for 5–60 min. The emergence of the N 1s peak in XPS indicates nitrogen incorporation in sapphire as soon as after 5 min of nitridation. AFM images show that the sapphire contained a high density of islands after 1 h of nitridation. A thin polycrystalline AlN layer was observed on the nitridated sapphire surface by TEM. Both the thickness of the AlN layer and the N 1s photoelectron peak intensity increase nonlinearly with respect to nitridation time. The nonlinear relationship between the thickness of the nitridated layer and the reaction time suggests the growth of the AlN layer follows...