[Show abstract][Hide abstract] ABSTRACT: Tandem solar cells using InxGa1-xN heterojunctions with silicon as the active junction were fabricated using gas-source molecular beam epitaxy (MBE) and by a novel deposition method incorporating an energetic nitrogen atom source. N-type InGaN layers were grown on p-Si(111) to evaluate predicted low-resistance tunnel junction properties. Ohmic behavior was observed, showing that these junctions can be used to connect the two pn subcells of an InGaN/Si tandem without the requirement of the heavily doped layers used in current multijunction cells. Undoped and Mg-doped films were grown by MBE on n-Si(111) using a AlN buffer layer. Depletion is observed on the Si side of the junction and efficiencies approaching 5% were measured for this “hybrid” cell design. Conditions for achieving depletion on the p-InGaN and producing a “single-junction” tandem cell are discussed.
[Show abstract][Hide abstract] ABSTRACT: The band gap energies of the In1−xAlxN alloys are continuously tunable across the solar spectrum, making them good candidates for high efficiency solar cells. In particular, multijunction solar cells could be fabricated entirely from different compositions of this one alloy system. From modeling experimental measurements of the optical absorption coefficient in alloys with 0 ≤ × ≤ 0.6, a band gap bowing parameter of 4.8 ± 0.5 eV is found for the alloy system. With this number, the alloy compositions for two and three junction tandem cells with maximum theoretical power conversion efficiencies can be determined. Two junction InAlN / Si solar cells are also promising structures for high efficiency solar cells, due to the properties of the interface between n-type InAlN and p-type Si, as well as the band gaps of the respective materials. The theoretical efficiency (detailed balance) for the In0.60Al0.40N / Si tandem cell is 41%.
Conference Record of the IEEE Photovoltaic Specialists Conference 05/2008; DOI:10.1109/PVSC.2008.4922884
[Show abstract][Hide abstract] ABSTRACT: In this work, the authors report step-flow growth mode of InN on  oriented GaN templates, using a production-style molecular beam epitaxy system, Veeco GEN200 registered , equipped with a plasma source. Using adaptive growth conditions, they have obtained a surface morphology that exhibits the step-flow features. The root mean squared roughness over an area of 5x5 m² is 1.4 nm with monolayer height terrace steps (0.281 nm), based on atomic force microscopy. It has been found that the presence of In droplets leads to defective surface morphology. From x-ray diffraction, they estimate edge and screw dislocation densities. The former is dominant over the latter. Micro-Raman spectra reveal narrow E² phonon lines consistent with excellent crystalline quality of the epitaxial layers. The Hall mobility of 1 m thick InN layers, grown in step-flow mode, is slightly higher than 1400 cm²/V s, while for other growth conditions yielding a smooth surface with no well-defined steps, mobility as high as 1904 cm²/V s at room temperature has been measured. The samples exhibit high intensity photoluminescence (PL) with a corresponding band edge that shifts with free carrier concentration. For the lowest carrier concentration of 5.6x10¹ cm³, they observe PL emission at 0.64 eV.
Journal of Vacuum Science & Technology A Vacuum Surfaces and Films 05/2008; 26(3):399-405. DOI:10.1116/1.2899412 · 2.32 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: p s s current topics in solid state physics c status solidi 1 Introduction With the increased availability of high-quality InN and In-rich InGaN samples, much pro-gress has been made in the understanding of the basic properties of these materials. The InGaN ternary alloy is of special interest since it spans a spectral range from the ul-traviolet to the infrared , taking into account the GaN band gap of 3.4 eV and the relatively recent discovery of a low band gap value of about 0.7 eV for InN . The high quality of InN and In-rich InGaN samples that have recently become available enables precise inspec-tion of theoretical models describing the physical proper-ties of these materials. In the present work, we will be in-terested in the influence of hydrostatic pressure on the op-tical properties of InN and In-rich InGaN. The hydrostatic pressure coefficient dE G /dp of the optical band gap is a ba-sic property of a semiconductor material. For nitride mate-rials, values of dE G /dp are are ~50 meV/GPa , ~40 meV/GPa , and ~25 meV/GPa  for AlN, GaN, and
[Show abstract][Hide abstract] ABSTRACT: We have demonstrated step-flow growth mode of InN, with monolayer height terrace steps (0.281 nm), using a production-style PA-MBE system, GEN200®. The surface morphology
exhibits the step-flow features on relatively large areas and the RMS roughness over an area of 5 x 5 μm2 is 1.4 nm. We also investigated the consequences of In droplets
formation during the growth and we have found that the vapor-liquid-solid growth mechanism generates defective layer areas underneath droplets that have been formed early in the growth process. The Hall mobility of 1μm thick InN layers, grown in such step-flow mode is slightly higher than 1400 cm2/Vs while for other growth conditions we have obtained mobility as high as 1904 cm2/Vs at room temperature. The samples exhibit high intensity photoluminescence spectra with a band edge that shifts with free-carrier concentration.
For the lowest carrier concentration of 5.6×1017 cm-3 we observe PL emission at ~0.64 eV.
physica status solidi (c) 05/2008; 5(6):1642-1644. DOI:10.1002/pssc.200778561
[Show abstract][Hide abstract] ABSTRACT: Temperature dependence of the reflectivity of InN with a carrier concentration of (3.5–4.7)×1017 cm3 is investigated from 50 to 750 cm−1 using infrared synchrotron radiation. E1 phonon is separately observed from plasma oscillation, and in the energy range below E1(TO), phonon polariton is observed up to 104 cm−1. The lifetime of the E1(TO) phonon is directly determined by the reflectivity measurements. From the temperature dependence of the lifetime, the E1(TO) phonon primarily decays into phonons with a renormalized frequency of 177 cm−1. From the plasma edge position the electron effective mass is estimated to be me⊥ = 0.076m0 for the intrinsic InN.
[Show abstract][Hide abstract] ABSTRACT: The variation in surface electronic properties of undoped c-plane InxAl1-xN alloys has been investigated across the composition range using a combination of high-resolution x-ray photoemission spectroscopy and single-field Hall effect measurements. For the In-rich alloys, electron accumulation layers, accompanied by a downward band bending, are present at the surface, with a decrease to approximately flatband conditions with increasing Al composition. However, for the Al-rich alloys, the undoped samples were found to be insulating with approximate midgap pinning of the surface Fermi level observed.
[Show abstract][Hide abstract] ABSTRACT: The valence band density of states VB-DOS of zinc-blende InN001 is investigated using a combination of high-resolution x-ray photoemission spectroscopy and quasiparticle corrected density functional theory. The zinc-blende VB-DOS can be characterized by three main regions: a plateau region after the initial rise in the DOS, followed by a shoulder on this region and a second narrow but intense peak, similar to other III-V and II-VI semiconductor compounds. Good general agreement was observed between the experimental and theo-retical results. Tentative evidence for an s-d coupling due to the interaction between valence-like N 2s states and semicore-like In 4d states is also identified. Measurements and calculations for wurtzite InN112 ¯ 0 are shown to yield a VB-DOS similar to that of zinc-blende InN, although the nonzero crystal field and different Brillouin zone shape in this case lead to a more complicated band structure which modifies the DOS. In adlayers terminating the InN112 ¯ 0 surface are also evident in the experimental VB-DOS, and these are discussed.
Physical Review B 03/2008; 77(11). DOI:10.1103/PhysRevB.77.115213 · 3.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The complex dielectric function DF of wurtzite InN and GaN as well as zinc blende GaN was measured by spectroscopic ellipsometry between 14 and 32 eV with synchrotron radiation. In this spectral region, the DF of InN and GaN originates from Ga 3d and In 4d core level transitions to unoccupied conduction-band states. The Ga 3d and In 4d electronic states are highly localized and show almost no dispersion. We use these core states as a reference in order to probe the conduction bands. For this purpose, the imaginary part is compared to the density of empty p-orbital-like electronic states located around the cation atomic site, as calculated by density-functional theory in the local density approximation. The constant splitting of absorption features in the DF is attributed to the spin-orbit splitting of the d states. d 5/2−3/2 is found to be 0.82 eV for the In 4d and 0.41 eV for the Ga 3d level, respectively. On wurtzite samples with the c axis in the surface plane, ellipsometry measurements give access to both independent dielectric tensor components and , respectively. The observed anisotropy is induced by a directional dependence of empty p states yielding a p -DOS density states different to the p -DOS.
[Show abstract][Hide abstract] ABSTRACT: InN has the largest electron affinity, 5.8 eV, of any known semiconductor. At its surface, the Fermi level is pinned ca. 0.9 eV above the conduction band edge, leading to an electron accumulation layer in n-type material and an inversion layer in p-type material. Recently, we have used capacitance-voltage measurements with an electrolyte contact to deplete the surface inversion layer in Mg-doped InN and observe space charge due to ionized acceptors . However, these measurements do not give information about the proportion of acceptors that are ionized, or about hole transport. Here, thermopower measurements are used to deduce the majority carrier type under the surface inversion layer in Mg-doped InN. Observation of a positive Seebeck coefficient provides direct and definitive evidence of mobile holes in InN:Mg. Temperature-dependent measurements from 200-300 K are consistent with degenerate conduction in the highly doped films. Modeling of the experimental data to determine the carrier effective masses and scattering mechanisms will be presented.  R. E. Jones et al., Phys. Rev. Lett. 96, 125505 (2006).
[Show abstract][Hide abstract] ABSTRACT: The core-level lineshape in photoemission spectra of InN is studied by high-resolution X-ray photoemission spectroscopy. The In 3d and N 1s core-levels are asymmetric, displaying a high binding energy tail which is attributed to inelastic losses to and/or screening by conduction band plasmons in the accumulation layer present at InN surfaces. The extent of the asymmetric tail decreases with decreasing surface Fermi level position associated with a lower density of electrons in the accumulation layer.
[Show abstract][Hide abstract] ABSTRACT: Time-resolved Raman spectroscopy on a subpicosecond time scale has been
used to study the phonon dynamics of both the A1(LO) and the
E1(LO) phonons in InN. From the temperature-dependence of
their lifetimes, we demonstrate that both phonons decay primarily into a
large wavevector TO phonon and a large wavevector TA/LA phonon
consistent with the accepted phonon dispersion relationship for wurtzite
InN. Their lifetimes have been found to decrease from 2.2 ps, at the low
electron-hole pair density of 5×1017cm-3 to
0.25 ps, at the highest density of
2×1019cm-3. Our experimental findings
demonstrate that carrier-density dependence of LO phonon lifetime is a
universal phenomenon in polar semiconductors.
Proceedings of SPIE - The International Society for Optical Engineering 02/2008; 6892:2-. DOI:10.1117/12.762319 · 0.20 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: This article reports on the study of microstructure and dislocation of InN films using high resolution x-ray diffraction grown on sapphire (0001) both by metalorganic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE). The mosaic tilt, twist, and correlation lengths of InN films are determined by using symmetrical and asymmetrical reflections as well as reciprocal space mapping. Deducing from these results, MBE-grown InN film exhibits the edge-type dislocations of 4.0×109 cm−2, which is about ten times higher than the density of screw-type dislocations. In MOCVD-grown InN sample, the edge-type dislocations density is as high as 2.1×1010 cm−2, and the screw-type dislocations density is 1.3×109 cm−2. They indicate that edge type is the predominant dislocation type in the InN films. By comparing the reported transmission electron microscopy results, the accuracy of evaluation for the dislocation density using the mosaic model is proved.