"Composition Dependence of Electronic and Optical Properties of GaAs1-x Nx Alloys:" LAURENT BELLAICHE, Su-Huai Wei, Alex Zunger, National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO 80401
The variation in band gap, atomic relaxations and wavefunction localization in GaAs1-xNx alloys are investigate as a function of composition x using a 512-atom supercell calculation with a carefully fitted empirical pseudopotential approach. We find (i) deep impurity levels for very small concentrations: the binding energies are about 50 meV from the Conduction Band Minimum for GaAs:N and, more remarkable, about 1 eV from the Valence Band Maximum for GaN:As, (ii) the persistence of such impurity-type levels even for concentrated alloys (x = 0.125), in good agreement with small supercell calculations using ab-initio all electron LDA approach, (iii) giant and composition dependent optical bowing coefficients (b > 8 eV). In spite of the fact that both bulk GaAs and GaN have rather large band gaps (1.5 and 3.2 eV respectively), we find a significant narrowing of the bandgap in the random alloy, in good agreement with the red shift of the photoluminescence observed recently for dilute GaAsN alloys; ordered alloys may even have negative gaps. Features (i)-(iii) are due to the unusually large differences of the atomic sizes and of the atomic pseudopotential depths between arsenic and nitrogen, leading to strong structural relaxations around the anions. Our analysis suggests that the optical and electronic properties of this alloy can be divided into two regions: (a) impurity-like region where the bowing coefficients are large and composition dependent and the wavefunctions are strongly localized, and (b) band-like region where the bowing coefficients are relatively small and the wavefunctions extended. The existence of these two regions is explained in terms of the existence of deep impurity levels. Connection with the percolation theory will be discussed.
Supported by BES/OER/DMS under contract no. DE-AC36-83CH10093.
"Exciton Structures in h-GaN Heteroexpitaxial Layers Grown on Sapphier Substrates by MOCVD:" S. CHICHIBU, Faculty of Science and Technology, Science University of Tokyo, 2641 Yamazaki, Noda, Chiba 278, Japan; A. Shikanai, T. Azuhata, T. Sota, Department of Electrical, Electronic, and Computer Engineering, Waseda University, 3-4-1 Ohkubo, Shinjuku, Tokyo 169, Japan; A. Kuramata, K. Horino, Fujitsu Laboratories Ltd., 10-1 Morimosato-Wakamiya, Atsugi, Kanagawa 243-01 Japan; S. Nakamura, Department of Research and Development, Nichia Chemical Industries Ltd., 491 Oka, Kaminaka, Anan, Tokushima 774, Japan
GaN and related nitrides are attracting much attention as the most suitable materials of optoelectronic devices such as light-emitting diodes and laser diodes operating in the blue to ultraviolet energy region. To improve device performances of GaN-based laser structure, it is desirable to understand biaxial strain dependencies of band structures and energies and electronic states such as exciton resonance energies, LO and TO exciton binding energies and effective masses, because the epi-layers essentially suffer certain amount of strain due to lack of homoepitaxial substrates. In addition, exciton states of nitrides are interesting not only for the gain calculation but also for understanding of the lasing mechanisms of quantum-well structures in terms of many-body effects, as is the case with widegap II-VI compounds. In this work, exciton resonance energies of h-GaN heteroepitaxial layers were measured as functions of temperature and residual lattice strain by means of photoreflectance (PR) measurements. Photoluminescence (PL) peak energies of exciton-related emissions were compared with PR results.
Samples measured in this study were undoped (n = 6x1016 cm-3) h-GaN epilayers, which were grown by metalorganic chemical vapor deposition. They were grown on sapphire (0001) substrates after depositing low-temperature GaN or AlN buffer layer. To control the degree of biaxial strain in GaN, GaN/AlGaN heterostructure was formed in some cases. The strain was estimated by x-ray bond methods. Free exciton resonance energies associated with transitions from three separate valence bands (A, B and C for [[Gamma]]9V, [[Gamma]]7V and [[Gamma]]7V respectively) to conduction ([[Gamma]]7c ) band were determined by the analysis of the PR spectra. All exciton resonance energies increased with increasing biaxial compressive strain, and the deformation potentials of them were determined. The deformation potential of the C exciton was found to be about twice that of A exciton. The data were analyzed theoretically using the Luttinger-Kohn type Hamiltonian for the valence bands under the in-plain biaxial stress. Occurrence of the anticrossing (exchange of band characteristics) of B and C valence bands in tensile biaxially strained h-GaN was suggested.
Free, the first excited, and bound exciton recombinations and E2-phonon sidebands of excitons were observed in low-temperature PL spectra of strained epilayers. Temperature-dependent PL measurements were carried out making a connection with the analysis of the PR spectra, and the PL spectra of weakly excited h-GaN were shown to be dominated by exciton-related emissions up to room temperature. Broadening of the value of the full width at half maximum of A-exciton emission was tentatively explained to be due to complex exciton-phonon coupling.
"Time-Resolved Photoluminescence Study of InGaN Single-Quantum-Well:" C.-K. SUN, S. Keller*, G. Wang, M. S. Minsky, B. Keller*, J. E. Bowers, S. DenBaars*, Department of Electrical and Computer Engineering, *Material Department, University of California, Santa Barbara, CA 93106
GaN based semiconductors have currently attracted a lot of attention for their potential applications as light emitters in the blue to UV range. Recently InGaN multiple quantum well laser diodes were demonstrated1 indicating a promising performance in the near future. However some important fundamental properties, such a radiative lifetime, of InGaN quantum wells (QWs) are still unknown. In this paper, we present the first time-resolved photoluminescence study of InGaN quantum wells. The low temperature free-carrier radiative lifetime was directly measured to be of the order of 250 ps at a generated carrier density of 1012 cm-2. The measured radiative lifetime is two times longer than the radiative lifetime measured in bulk GaN samples2 at low temperature. In this study we also report on luminescence lifetime measured at room temperature for the first time.
A 390 Å thick InGaN:Si layer of graded composition was first grown on a 1.8 um thick GaN film before the growth of a 50 Å thick In0.13Ga0.87N QW. After the QW growth, a 45 Å-thick In0.04Ga0.96N:Si layer was grown with a 0.1 um thick GaN cap layer. Time resolved photoluminescence (TRPL) was performed using a frequency tripled Ti:Sapphire laser at a wavelength of 255 nm. The time resolved measurement has a system resolution around 20 ps. Photoluminescence (PL) studies show that the QW luminescence is centered at 399 nm at 22 K with 5.3 nm FWHM linewidth and centered at 401 nm wavelength at 300K with 12 nm FWHM linewidth. Figure 1 shows the measured TRPL at 7.1 K with excited carrier density ~1x1012 cm-1 at the luminescence peak wavelength. The TRPL shows a bimolecular decay. A convolution fit was used to extract the constant B*n, where n is the carrier density. A good fit was obtained with B*n = 3x109 s-1 (dotted line). A single exponential fit (dashed line) was also employed to extract the time constant. Figure 2 shows the measured time constant using a single exponential fit as a function of temperature at the luminescence peak. As the temperature is raised form 7 to 21 K, the measured lifetime increases from 230 ps to 260 ps. This data indicates that at low temperature, the bandedge luminescence is dominated by free carrier radiative recombination. At temperatures higher than 20K, the measured lifetime decreases down to 130 ps at room temperature.
1Shuji Nakamura, et al. Jpn. J. Appl. Phys. 35, L74 (1996).
2C. I. Harris et al., Appl. Phys. Lett. 67, 840 (1995).
"Photoluminescence Characteristics of GaN/InGaN/GaN Quantum Wells:" I.K. SHMAGIN, J. Muth, R.M. Kolbas, S. Krishnankutty, Electrical and Computer Engineering Department, North Carolina State University, Raleigh, NC 27695; S. Keller, B.P. Keller, A.C. Ambare, L.A. Coldren, U.K. Mishra, S.P. DenBaars, Electrical & Computer and Material Departments, University of California, Santa Barbara, CA 93106
Blue light emitting diodes and lasers developed recently in the III-V nitride material system have consisted of InGaN/GaN heterojunctions and quantum wells as the active layers. In order to fully understand the emission characteristics of these active layers we have carried out a detailed analysis of the GaN/InGaN/GaN quantum wells using CW and pulsed photoluminescence.
The InGaN single quantum wells were grown by atmospheric pressure metal organic chemical vapor deposition (MOCVD). The quantum wells consisted of a 2 um GaN layer deposited at 1060deg.C, then a 400 Å In0.04Ga0.96N layer, the In0.15Ga0.85N quantum well 20-80 Å thick, a 50 Å In0.04Ga0.96N layer, and finally a 0.1 mm GaN layer deposited at 1060oC. The growth temperatures for the InGaN layers were between 700-760oC. A reference sample consisting of a thick 0.1 um In0.15Ga0.85N layer deposited on a 2.4 um GaN layer was also grown. Room temperature and 77 K photoluminescence measurements were performed using a CW Ar+ ion laser (305 nm) and a frequency-tripled, pulsed, mode-locked Ti:sapphire laser (280 nm). Both edge and surface emitting configurations were studied.
Photoluminescence spectra from the quantum wells were blue shifted with respect to the band-edge emission from the In0.15Ga0.85N reference sample. The confined energy states in the quantum well were calculated using a finite square well model. Based on a comparison to the calculations, the emission spectra from the quantum wells were identified as the n=1 confined particle transition. Differences between the measured value of peak emission energy from the quantum wells and the calculated value of the n=1 confined particle transitions were attributed to strain, arising due to the significant lattice mismatch between InGaN and GaN. Data representative of these observations will be presented along with a discussion of quantum confinement effects, strain induced effects and critical thickness considerations.
The authors wish to acknowledge the support of the U.S. Army Research Office through a contract supervised by Dr. John Zavada and the support of the ARPA Optoelectronics Technology Center.
"Growth and Properties of AlGaInN/InGaN Heterostructures:" E.L. PINER, F.G. McIntosh*, J. C. Roberts*, K. Boutros*, N.A. El-Masry, S.M. Bedair*, Materials Science and Engineering Department, *Electrical and Computer Engineering Department, North Carolina State University, Raleigh, NC 27695-7916
AlGaN/InGaN quantum well heterostructures suffer from a high lattice mismatch and thus are difficult to grow with high values of x (%In). Such limitation can be avoided if AlGaN is replaced by the more versatile AlyInxGa1-x-yN where the band gap and lattice constant can be independently varied by changing the values of x and y. We will report on AlyInxGa1-x-yN/InxGa1-xN lattice matched quantum wells and AlGaInN/GaN heterostructures.
The quaternary alloy is grown in an atmospheric pressure MOCVD in the temperature range of 780-800oC as a compromise for the growth requirements of Al and In compounds. The films were grown in the composition range 0< y < 0.3 and 0< x < 0.2. The film composition, studied by EDS and the lattice constants by x-ray diffraction, were found to follow Vegard's law indicating solid solubility at least in the composition range studied. Room temperature PL showed bandedge emissions that can be predicted from the bandgaps of the constituent ternary alloys. For high values of y (AlN %) PL was dominated by deep levels. An AlyInxGa1-x-yN/InxGa1-xN heterostructure was grown with lattice mismatch of 0.3% and the corresponding mismatch for the AlGaN/InGaN with same values of x is about 1.8%. The FWHM of the PL spectrum of the AlyInxGa1-x-yN/InxGa1-xN is at least comparable to the corresponding mismatch for the AlyGa1-yN/InxGa1-xN structure. It is expected that for values of x>0.2 further improvement in the properties of the lattice matched quaternary/ternary films will surpass that of the ternary/ternary ones. Also, an almost matching AlGaInN/GaN structure was achieved.
We will report on structural, electrical and optical properties of these quaternary films and their corresponding lattice matched structure with GaN and InGaN and their potential application in both optical and microwave devices.
"Luminescence and Raman Properties of GaN Epilayers Grown on Sapphire and 6H-SiC:" B.J. SKROMME, J.W. Hutchins, H. Zhao, Department of Electrical Engineering and Center for Solid State Electronics Research, Arizona State University, Tempe, AZ 85287-5706; H.S. Kong, M.T. Leonard, G.E. Bulman, Cree Research Inc., 2810 Meridian Pkwy., Ste. 176, Durham, NC 22713; C.R. Abernathy, S.J. Pearton, Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611
Gallium nitride and related materials are currently of considerable interest for short wavelength light-emitting devices. The large lattice and thermal mismatches between GaN and the typical substrates, sapphire or 6H-SiC, produce considerable strain in most as-grown layers. The effects of strain on luminescence, reflectance, and Raman properties of the materials have been investigated only to a very limited degree for these systems to date. We describe a detailed optical spectroscopic study of residual (mainly thermal mismatch) strain in GaN grown on both c-axis sapphire and (0001) 6-H SiC by either metalorganic chemical vapor deposition, or gas-source molecular beam epitaxy using trimethylgallium and ammonia. The experimental techniques included low-temperature (1.7 K and above) and room temperature photoluminescence (PL), reflectance and polarized Raman scattering. The [[Gamma]]9, [[Gamma]]71 and [[Gamma]]72 excitons were clearly resolved in reflectance and confirmed by observation of corresponding PL peaks. Neutral donor-bound exciton peaks and phonon replicas of the free and bound excitons were observed and studied as a function of excitation intensity and temperature to confirm their identification. Bound exciton linewidths were all less than 4 meV, confirming good quality material. A detailed analysis of the exciton splittings has been carried out using the Pikus-Bir strain Hamiltonian for the wurtzite structure and fits to the experimental data. The signs of the thermal strains and corresponding exciton shifts are found to be opposite for SiC substrates compared to sapphire, as expected based on thermal expansion coefficient data. Strain effects are also analyzed for the bound excitons, and the donor-bound exciton is shown to shift from e.g. 3.459 eV for SiC substrates to 3.475 eV for sapphire substrates, a considerable difference. A possible two-electron replica of the donor-bound exciton emission has been observed in both types of material; its separation from the principal bound exciton line is exactly the same (22.0 meV) in both cases. Sharp luminescence peaks are also observed at 3.366 and 3.306 eV for GaN on sapphire substrates, which have been frequently attributed to bound excitons in GaN. We show that they actually originate from deep level PL involving impurities in the sapphire substrate. The Raman spectra also show clear evidence of differing strain shifts in the E2 peak position, which differs by as much as 2 cm-1 for the different substrates.
"Si, Mg, P and Mg/P Implantation and Annealing of Semi-Insulating and n-type MOCVD GaN:" B. MOLNAR, Andrew Edwards, M.V. Rao, A. Wickenden, M. Schurman, Z.C. Feng, R.A. Stall, Naval Research Laboratory, 4555 Overlook Ave., Washington, D. C. 20375-5347; ECE Department, George Mason University, Fairfax, VA 22030; EMCORE Corporation, 394 Elizabeth Ave., Somerset, NJ 08873
The application of ion-implantation (for doping and damage) to GaN makes the fabrication of new device structures possible. In this study, we compared the results of ion implantation in MOCVD GaN grown in different systems in order to establish if the unknown background doping in influencing the impact activation. Implants were performed in both semi-insulating and n-type (1016-1018 cm-3) GaN layers grown on [[alpha]]-Al2O3. The background doping uniformity across the wafer was evaluated by measuring the electrical properties of the n-type layer across 2" diameter wafers. We observed variations of 30% in the resistivity and 20% in the mobility from the center to the edge of the wafer. Thermal stability of GaN has also been evaluated in the temperature range of 500oC to 1200oC in N2 ambient. No Ga liquid formation was observed up to 1000oC. The Ga liquid formation limits the maximum annealing temperature to 1150oC for 120 s. The high temperature annealing without implantation introduced n-type conductivity in SI GaN. The stability of implantation damage caused by P (which was used for isolation) in n-type material was also evaluated as a function of annealing temperature. The implanted P concentration level is 1019 cm-3. The damage is persistent electrically even after annealing at 1000oC for 30 min.
We implanted Si, Mg, and Mg/P to 1019 cm-3 range at RT and elevated temperatures up to 600oC. As expected, the damage is less at elevated temperatures. A maximum Si activation of 65% in the material annealed at 1150oC was observed. We have seen a variation of activation between 15-65% (for the same implant and anneal conditions) for the material grown in two different systems. We could not measure any acceptor activation in Mg implanted samples.
"Characteristics of p-n Junctions in GaN-Based Light Emitting Diodes:" P. PERLIN, J. Mu, P. G. Eliseev, P. Sartori, M. Osinski, Center for High Technology Materials, University of New Mexico, Albuquerque, NM 87131-6081
In the last two years, a milestone event in the optoelectronic industry was a fabrication and further commercialization of blue light emitting diodes based on GaN/InGaN/AlGaN heterostructures grown on sapphire substrates. These devices, developed by Nichia Chemical Industries, proved to be efficient and reliable sources of radiation in the short-wavelength part of the spectrum. The Nichia LEDs were followed recently by a new product from Cree Research company which differs mostly in the use of SiC as a substrate material. In this paper, we concentrate on investigations of the electrical characteristics of p-n heterojunctions in the III-N based LEDs. The fingerprint of the electron transport through these junctions at small and moderate voltages is strong presence of a tunneling process, as evidenced by practically temperature-independent slopes of current-voltage characteristics in semi-logarithmic representation. The normal diffusion current component becomes important only at voltages exceeding 3 V. Similarity of results obtained for Nichia and Cree LEDs indicates that the importance of the tunneling mechanism can be ascribed to a high density of dislocations, common to both devices. Interestingly, the light emitted by the Nichia diodes is proportional to the tunneling current, showing that this mechanism directly contributes to optical transitions. In order to describe quantitatively the electrical properties of the junction, we tried to use the multistep Riben-Feucht model1. However, we have found out that the Riben-Feucht model fails, especially in describing the temperature dependence of the tunneling current. A deeper understanding of the p-n junction physics in these materials is required in order to develop a new model which would quantitatively explain the obtained results.
References: 1. A.R. Riben and D.L. Fuecht , Solid-State Electron. 9, 1055 (1966)
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