Luminescence properties of ZnSe films grown by hot wall epitaxy

Received: 10 June 1996/Accepted: 20 October 1996     

Estimation of InN phase inclusion in InGaN films grown by MOVPE

The amount of InN included in InGaN films grown by MOCVD (metalorganic chemical vapor deposition) was estimated by X-ray diffraction measurement technology. The In compositions in our InGaN films are measured as 0.1–0.34 by X-ray 2θ scan using Vegard’s law. The inclusion of InN in InGaN layers was obtained as 0.0684–2.6396% by measuring the ratio of the integrated intensity of the InN (0002) peak to that of the InGaN (0002) peak in X-ray rocking curves. The theoretical diffraction intensities from InN and InGaN have been calculated according to the X-ray-diffraction theory. The values of the InN inclusion for all our samples were less than 3%, which indicated that the degree of phase separation of the samples was low. It was also found that the flow rate of N₂ carrier gas and the operation pressure strongly affected the InN inclusion in InGaN. Received: 20 November 2000 / Accepted: 16 May 2001 / Published online: 27 June 2001     

Growth of highly disordered InGaP on (100) GaAs by molecular beam epitaxy with a GaP decomposition source

High-quality, lattice-matched InGaP on exact (100) GaAs was successfully grown by molecular beam epitaxy with a GaP decomposition source. The ordering parameter (η) of the InGaP is investigated as a function of the growth temperature. η is as low as 0.22 and almost insensitive to the growth temperature below 460 °C. It increases abruptly around 475 °C and has a maximum value of 0.35 at ≈490 °C. Double crystal X-ray diffraction and a low-temperature photoluminescence spectrum reveal that the present growth method is robust and provides better quality InGaP compared to other state-of-the-art growth technologies. Received: 20 November 2000 / Accepted: 27 January 2001 / Published online: 21 March 2001     

The critical thickness of silicon-germanium layers grown by liquid phase epitaxy

Silicon-germanium layers are grown from metallic solution on (100) and (111) silicon substrates. On (111) Si, coherently strained dislocation-free SiGe layers are obtained with thicknesses larger than predicted by the current models of misfit-induced strain relaxation. A comprehensive characterisation by imaging, diffraction, and analytical electron microscopy techniques is carried out to determine the critical thickness, study the onset of plastic relaxation, and explain the particular growth mechanisms leading to an unexpectedly high thickness of elastically strained SiGe layers. A vertical Ge concentration gradient and the formation of step edges on the layers, where lateral strain relaxes locally, explain the high critical thickness. The model of Matthews and Blakeslee is modified in order to match the experimental observations for solution-grown SiGe layers. Received: 29 July 1999 / Accepted: 29 July 1999 / Published online: 27 October 1999     

Lateral variation of quantum-well confinement energy in triangular-shaped dot-like structures grown by molecular beam epitaxy on patterned GaAs (311)A substrates

Received: 13 Oktober 1997/Accepted: 25 January 1998     

Self-organized formation of shell-like InAs/GaAs quantum dot ensembles

Formation of a multimodal quantum dot (QD) ensemble by strained layer epitaxy of InAs on GaAs near the critical value for the onset of the 2D-3D transition is studied. Reflection anisotropy spectroscopy is employed to confirm that a smooth surface is maintained during strained layer growth prior to QD formation. Instantaneous capping after deposition leads to InAs quantum wells with some thickness flucuations. Multimodal QD InAs ensembles form after an at least short growth interruption prior to cap layer deposition. The QDs consist of pure InAs with heights varying in steps of complete InAs monolayers. Related exciton energies indicate a simultaneous increase of both height and lateral extension, i.e. a shell-like increase of sizes. The formation of the multimodal QD ensemble is described by a kinetic approach. A growth scenario is presented where QDs having initially shorter base length stop vertical growth at a smaller height, accounting for the experimentally observed shell-like sub-ensemble structure.     

Optical gain of interdiffused GaInNAs/GaAs quantum wells

A self-consistent model for the band structure and optical gain spectra in interdiffused Ga_xIn_1-xN_0.04As_0.96/GaAs single quantum wells are studied theoretically using Fick’s Law and the Fermi Golden Rule. Due to quantum-well interdiffusion, the peak gain and its peak vary with the annealing time. Our results show that the interdiffusion technique can be used to tune the operation wavelength for multi-wavelength applications without degradation of device performance. Received: 18 April 2001 / Accepted: 19 September 2001 / Published online: 20 December 2001     

InGaP/GaAs heterojunction bipolar transistor grown by solid-source molecular beam epitaxy with a GaP decomposition source

Lattice-matched InGaP epilayers on GaAs (001) and InGaP/GaAs heterojunction bipolar transistors (HBTs) were successfully grown by solid-source molecular beam epitaxy (SSMBE) with a GaP decomposition source. A 3 μm thick InGaP epilayer shows that low temperature photoluminescence (PL) peak energy is as large as 1.998 eV, full width at half maximum (FWHM) is 5.26 meV, which is the smallest ever reported, and X-ray diffraction (XRD) rocking curve linewidth is as narrow as that of GaAs substrate. The electron mobilities at room temperature of nominally undoped InGaP layers obtained by Hall measurements are comparable to similar InGaP epilayer grown by solid-source molecular beam epitaxy (SSMBE) with other sources or other growth techniques. The large area InGaP/GaAs HBTs show very good Dc characteristics.     

Passivation of GaAs surface by atomic-layer-deposited titanium nitride

The suitability of titanium nitride (TiN) for GaAs surface passivation and protection is investigated. A 2-6-nm thick TiN passivation layer is deposited by atomic layer deposition (ALD) at 275 ° C on top of InGaAs/GaAs near surface quantum well (NSQW) structures to study the surface passivation. X-ray reflectivity measurements are used to determine the physical properties of the passivation layer. TiN passivation does not affect the surface morphology of the samples, but increases significantly the photoluminescence intensity and carrier lifetime of the NSQWs, and also provides long-term protection of the sample surface. This study shows that ALD TiN coating is a promising low-temperature method for ex situ GaAs surface passivation.     

In situ optical monitoring of metalorganic vapor phase epitaxy growth of C-doped GaAs

20 cm^-3 and above) induce oscillations in the reflected intensity. The evolution of the layers morphology is shown to depend on two effects: (i)the locally high surface concentration of carbon which blocks locally the growth and hence induces holes at the surface, (ii)the occurrence of dislocations at thicknesses larger than the critical thickness which are revealed chlorides produced by the decomposition of CCl₄ (the carbon precursor) and form deep etch pits. Received: 13 February 1998 / Accepted: 26 October 1998     

Optimum Indium Concentration for Growth of 1.3μmInAs/InxGa1-xAs Quantum Dots in a Well

Five InAs/InxGa1-xAs quantum dots in a well （DWELL） with different indium concentration x are grown by solid source molecular beam epitaxy. The high quantum dot density is observed in the InAs/In0.3Ga0.7As DWELL. The photoluminescence （PL） experiments indicate that the ground state peaks of InAs/In0.15 Ga0.85As and InAs/In0.22 Ga0.78As DWELLs shift to 1.31 and 1.33μm, respectively. The optical properties are investigated by using the PL and piezoreflectance spectroscope methods. An abnormal blue shift has been observed with the further increase of x from 0.22 to 0.30.     

Recombination mechanism of photoluminescence in InN epilayers

We report an investigation of the recombination mechanism for photoluminescence (PL) in InN epilayers grown by molecular beam epitaxy and metal-organic chemical vapor deposition with a wide range of free electron concentrations from 3.5×10¹⁷-5×10¹⁹ cm⁻³. We found that the PL spectra are strongly blueshifted with increasing excitation intensity. For all the samples studied, the exponent of the relationship between the integrated PL intensity and the excitation intensity is very close to unity and independent of the temperature. By assuming Gaussian fluctuations of the random impurity potential, calculation based on the ‘free-to-bound’ recombination model can be used to interpret our results very well and it correctly reproduces the development of the total PL peak shift as a function of carrier concentration. It is concluded that the PL transition mechanism in InN epifilms can be characterized as the recombination of free electrons in the conduction band to nonequilibrium holes in the valence band tail.     

Optical anisotropy in nonspherical quantum dots

Received: 10 April 1997/Accepted: 12 June 1997     

Comparative studies on structural and optical properties of ZnO films grown on c-plane sapphire and GaAs (001) by MOCVD

ZnO thin films were grown on c-plane sapphire and GaAs (001) substrates by metalorganic chemical vapor deposition. Atomic force microscopy and double-crystal X-ray diffractometry were utilized to investigate the structural properties of the ZnO films. The optical properties of ZnO films were also investigated in terms of time integrated and resolved photoluminescence (TIPL and TRPL). Large hexagonal crystallites and better crystalline quality were observed from the ZnO film on sapphire. Also, both the TIPL and TRPL showed a significant difference as the substrate changed. In particular, a detected sharp contrast in the result of TRPL measurement is due to the different defect structure and the lattice strain and stress of ZnO films on different substrates.     

Optical diffuse reflectance spectra of GaSb nanocrystals embedded in SiO2 matrix by radio-frequency magnetron co-sputtering

Nanocrystalline GaSb embedded in SiO₂ films was grown by radio-frequency magnetron co-sputtering. X-ray diffraction pattern and transmission electron microscopy (TEM) confirm the existence of GaSb nanocrystals in the SiO₂ matrix. The average size of GaSb nanoparticles is in the range of 3 to 11 nm. Diffuse reflectance spectra were used to characterize the small change of the band gap of the semiconductor. The diffuse reflectance spectra shows that the absorption peaks have a large blueshift of about 4.0 eV of the absorption relative to that of bulk GaSb. It has been explained by quantum confinement effects. Room temperature optical transmission spectra show that the absorption edge exhibits a very large blueshift of about 2.1 eV with respect to that of bulk GaSb in agreement with quantum confinement. Received: 28 July 1999 / Accepted: 27 October 1999 / Published online: 1 March 2000     

Influence of CH3COO on the room temperature photoluminescence of ZnO films prepared by CVD

ZnO films with strong c-axis-preferred orientation have been prepared by a single source chemical vapor deposition technique using zinc acetate as source material at the growth temperature of 230 °C. The strong UV and blue emissions were observed in the photoluminescence spectra of as-grown films. A small quantity of residual zinc acetate was reserved on the surface of as-grown ZnO films and the emission mechanism of blue luminescence was nearly related to the CH₃COO^- of unidentate type. The blue emission disappeared and the green emission appeared after annealing treatment. The green emission is related to the singly ionized oxygen vacancies.     

Microscopic theory of optical dephasing in semiconductors

Dephasing, i.e. the decay of the optical interband polarization in a semiconductor results from destructive interference effects between different microscopic contributions. For a system without any disorder it is shown that the many-body Coulomb correlations lead to excitation-induced dephasing which becomes increasingly important at elevated excitation levels. The effect of disorder-induced dephasing is analyzed for low excitation levels, where the combined influence of excitonic, biexcitonic, and disorder scattering contributions lead to a temporal decay of the four-wave-mixing signal. Received: 30 March 2000 / Accepted: 2 September 2000 / Published online: 12 October 2000     

Enhanced light output of InGaN LEDs with a roughened p-GaN surface using different TMGa flow rates in p-AlGaN layer

We have demonstrated the enhancement of light output of InGaN-based blue light-emitting diodes (LEDs) using different trimethylgallium (TMGa) flow rates in the growth of p-AlGaN epilayer to facilitate a rougher p-GaN surface. It is found that higher output power can be achieved from the LEDs with rougher surface morphologies when the TMGa flow rate (R_TMGa) is increased up to 60 sccm during p-Al_0.05Ga_0.95N epilayer growth. Such a rough surface obtained at higher R_TMGa is attributed to the fact that the vertical growth rate is faster than the lateral growth rate, thus, leading to the facet of crystal growth focuses mainly in the vertical direction. The output power of devices biased at 20 mA is 15.4, 15.9, 17.5, and 18.9 mW for TMGa flow rates of 10, 20, 40, and 60 sccm, respectively.     

Photoreflectance characterization of InAs/GaAs self-assembled quantum dots grown by ALMBE

We report on a photoreflectance investigation in the 0.8-1.5 eV photon energy range and at temperatures from 80 to 300 K on stacked layers of InAs/GaAs self-assembled quantum dots (QDs) grown by Atomic-Layer Molecular Beam Epitaxy. We observed clear and well-resolved structures, which we attribute to the optical response of different QD families. The dependence of the ground state transition energy on the number of stacked QD layers is investigated and discussed considering vertical coupling between dots of the same column. It is shown that Coulomb interaction can account for the observed optical response of QD families with different morphology coexisting in the same sample. Received 17 November 1999     

High-Reflectivity A1GaN/A1N Distributed Bragg Reflector in Ultraviolet Region

Thirty-pair Alo.3 Gao.T N/A1N distributed Bragg reflectors centred at 32Ohm are designed and grown on sapphire substrates by metalorganic chemical vapour deposition. No cracks are observed in the main area of the 2-inch wafer except for about 4 mm margin under an optical microscope. Regular stack of alternating layers is shown by scanning electron microscopy. Clear two-dimensional growth steps and very low surface roughness are shown by atomic force microscopy （AFM）. Well-defined periodicity is shown by high resolution x-ray diffraction. High refiectivity of 93% at 313nm with a bandwidth of 13nm is obtained.