A. Botchkarev

Deep centers in n-GaN grown by reactive molecular beam epitaxy

Deep centers in Si-doped n-GaN layers grown by reactive molecular beam epitaxy have been studied by deep-level transient spectroscopy as a function of growth conditions. Si-doped GaN samples grown on a Si-doped n+-GaN contact layer at 800u2009°C show a dominant trap C1 with activation energy ET=0.44u2009eV and capture cross-section σT=1.3×10−15u2009cm−2, while samples grown at 750u2009°C on an undoped semi-insulating GaN buffer show prominent traps D1 and E1, with ET=0.20u2009eV and σT=8.4×10−17u2009cm2, and ET=0.21u2009eV and σT=1.6×10−14u2009cm2, respectively. Trap E1 is believed to be related to a N-vacancy defect, since the Arrhenius signature for E1 is very similar to the previously reported trap E, which is produced by 1-MeV electron irradiation in GaN materials grown by both metalorganic chemical-vapor deposition and hydride vapor-phase epitaxy.

Deep level defects in n-type GaN grown by molecular beam epitaxy

Deep-level transient spectroscopy has been used to characterize electronic defects in n-type GaN grown by reactive molecular-beam epitaxy. Five deep-level electronic defects were observed, with activation energies E1=0.234±0.006, E2=0.578±0.006, E3=0.657±0.031, E4=0.961±0.026, and E5=0.240±0.012 eV. Among these, the levels labeled E1,u2002E2, and E3 are interpreted as corresponding to deep levels previously reported in n-GaN grown by both hydride vapor-phase epitaxy and metal organic chemical vapor deposition. Levels E4 and E5 do not correspond to any previously reported defect levels, and are characterized for the first time in our studies.

PIEZOELECTRIC EFFECTS ON THE OPTICAL PROPERTIES OF GAN/ALXGA1-XN MULTIPLE QUANTUM WELLS

Piezoelectric effects on the optical properties of GaN/AlGaN multiple quantum wells (MQWS) have been investigated by picosecond time-resolved photoluminescence (PL) measurements. For MQWS with well thickness 30 and 40 the excitonic transition peak positions at 10 K in continuous wave (CW) spectra are red-shifted with respect to the GaN epilayer by 17 meV and 57 meV, respectively. The time-resolved PL spectra of the 30 and 40 well MQWS reveal that the excitonic transition is in fact blue-shifted at early delay times due to quantum confinement of carriers. The spectral peak position shifts toward lower energies as the delay time increases and becomes red-shifted at longer delay times. We have demonstrated that the results described above is due to the presence of the piezoelectric field in the GaN wells of GaN/AlGaN MQWS subject to elastic strain together with screening of the photoexcited carriers. By comparing experimental and calculation results, we conclude that the piezoelectric field strength in GaN/Al.15G~.85N MQWS has a lower limit value of about 560 kV/cm: The electron and hole wave function distributions have also been obtained. The implication of our findings on the practical applications of GaN based optoelectronic devices is also discussed.

Optical modes within III-nitride multiple quantum well microdisk cavities

Optical resonance modes have been observed in optically pumped microdisk cavities fabricated from 50 A/50 A GaN/AlxGa1−xN(x∼0.07) and 45 A/45 A InxGa1−xN/GaN(x∼0.15) multiple quantum well structures. Microdisks, approximately 9 μm in diameter and regularly spaced every 50 μm, were formed by an ion beam etch process. Individual disks were pumped at 300 and 10 K with 290 nm laser pulses focused to a spot size much smaller than the disk diameter. Optical modes corresponding to (i) the radial mode type with a spacing of 49–51 meV (both TE and TM) and (ii) the Whispering Gallery mode with a spacing of 15–16 meV were observed in the GaN microdisk cavities. The spacings of these modes are consistent with those expected for modes within a resonant cavity of cylindrical symmetry, refractive index, and dimensions of the microdisks under investigation. The GaN-based microdisk cavity is compared with its GaAs counterpart and implications regarding future GaN-based microdisk lasers are discussed.

Well-width dependence of the ground level emission of GaN/AlGaN quantum wells

We have performed a systematic investigation of GaN/AlGaN quantum wells grown on different buffer layers (either GaN or AlGaN) in order to clarify the role of strain, structural parameters, and built-in field in determining the well-width dependence of the ground level emission energy. We find that identical quantum wells grown on different buffer layers exhibit strong variation of the ground level energy but similar well-width dependence. The data are quantitatively explained by an analytic model based on the envelope function formalism which accounts for screening and built-in field, and by a full self-consistent tight binding model.

Surface roughness of nitrided (0001) Al2O3 and AlN epilayers grown on (0001) Al2O3 by reactive molecular beam epitaxy

Nitridation of c-plane sapphire followed by deposition of a low temperature AlN or GaN buffer is commonly employed in the growth of GaN-based structures which have received a great deal of attention recently. In order to gain some needed insight, we undertook an investigation of nitridation of sapphire followed by the subsequent growth of AlN buffer layers in a reactive molecular beam epitaxy environment. Atomic force microscopy was used to characterize the surface roughness of samples after exposure to various nitridation conditions. Nitridation at higher temperatures was found to yield smoother substrate surfaces possibly due to smoothing of scratches introduced during substrate preparation. Incorporation of nitrogen into sapphire surfaces during the nitridation process was verified using x-ray photoelectron spectroscopy by observing the development of the N 1s peak with nitridation time. The surface roughness of AlN layers deposited on these nitrided surfaces was found to increase dramatically with thi...

Effect of Ammonia Flow Rate on Impurity incorporation and Material Properties of Si-Doped GaN Epitaxial Films Grown by Reactive Molecular Beam Epitaxy

Effect of ammonia flow rate on the impurity incorporation and material properties of Si-doped GaN films grown by reactive molecular beam epitaxy (RMBE) process is discussed. It appears that the ammonia flow rate has a marginal effect on the incorporation of impurities into the Si-doped GaN films except there was a little decrease in O and Si with increasing ammonia flow rate when the Si concentration in the film is higher than 1018u2009cm−3. Electron Hall mobility of Si-doped GaN films grown by RMBE varies with ammonia flow rate used during film growth. From deep level transient spectroscopy (DLTS) measurements for Schottky diodes grown with different ammonia flow rates, one deep trap (C1) particular to the RMBE films was found. The concentration of C1 trap was found to be the lowest in the sample grown with the condition leading to the highest electron Hall mobility within the scope of this experiment. In addition to the DLTS result, other characterization techniques used (x-ray diffraction, cross-sectional ...

Plasma heating in highly excited GaN/AlGaN multiple quantum wells

Time-resolved photoluminescence (PL) spectroscopy was used to investigate carrier distributions in a GaN/AlGaN multiple quantum well (MQW) sample under high excitation intensities necessary to achieve lasing threshold. Room temperature PL spectra showed optical transitions involving both confined and unconfined states in the quantum well structure. Analysis of the experimental results using a microscopic theory, indicates that at high excitation the carrier distributions are characterized by plasma temperatures which are significantly higher than the lattice temperature. The implications of our findings on GaN MQW laser design are also discussed.

Optical properties and resonant modes in GaN/AlGaN and InGaN/GaN multiple quantum well microdisk cavities

Optical resonance modes have been observed in optically pumped microdisk cavities fabricated from 50 angstroms/50 angstroms GaN/AlxGa1-xN (x approximately 0.07) and 45 angstroms/45 angstroms InxGa1-xN/GaN (x approximately 0.15) multiple quantum well structures. Microdisks, approximately 9 micrometers in diameter and regularly spaced every 50 micrometers , were formed by ion beam etch process. Individual disk was pumped from 10 K to 300 K with 290 nm laser pulses focused to a spot size much smaller than the disk diameter. Optical properties of these microdisks have been studied by picosecond time-resolved photoluminescence (PL) spectroscopy. From cw PL emission spectra, optical modes corresponding to (1) the radial mode type with a spacing of 49 - 51 meV (both TE and TM) and (2) the Whispering gallery mode with a spacing of 15 - 16 meV were observed in the GaN-based microdisk cavities. The spacings of these modes are consistent with theoretical calculation. The implications of our results to III-Nitride microdisk lasers are discussed.

Piezoelectric effects on the dynamics of optical transitions in GaN/AlxGa1-xN multiple quantum wells

Piezoelectric effects on the dynamics of optical transitions in GaN/AlGaN multiple quantum wells (MQWs) have been investigated by picosecond time-resolved photoluminescence (TRPL) measurements. TRPL spectra of the 40 angstroms well MQWs reveal that the excitonic transition is in fact blueshifted at early delay times due to quantum confinement of carriers. The spectral peak position shifts toward lower energies as the delay time increases and becomes redshifted at longer delay times. By comparing experimental and calculation results a low limit of the piezoelectric field strength of about 460 kV/cm in GaN/Al0.15Ga0.85N MQWs and the electron and hole wave functions have been obtained. Temporal response of the excitonic transitions of the GaN/AlGaN MQWs depends on the well width. The recombination lifetimes of the 20 angstroms well MQWs decreases monotonously with an increase of emission energy. However, emission energy dependence of the lifetime on 30, 40, 50 angstroms well MQWs which shows a similar behavior as the cw PL spectrum, is quite different from that of 20 angstroms well MQWs. It has been demonstrated that the results described above are due to the presence of the piezoelectric field in the GaN wells of GaN/AlGaN MQWs subject to elastic strain together with screening of the photoexcited carriers and Coulomb interaction.