William E. Fenwick

Characterization of InN layers grown by high-pressure chemical vapor deposition

Structural and optical properties of indium nitride (InN) layers grown by high-pressure chemical vapor deposition (HPCVD) on sapphire and GaN epilayers have been studied. HPCVD extends processing parameters beyond those accessible by molecular beam epitaxy and metal organic chemical vapor deposition, enabling the growth of epitaxial InN layers at temperatures as high as 1150K for reactor pressures around 15bars, leading to vastly improved material properties. InN layers grown on GaN(0002) epilayers exhibit single-phase InN(0002) x-ray diffraction peaks with full width at half maximum (FWHM) around 430arcsec. Optical characterization of the InN layers by infrared (IR) reflectance reveals free carrier concentrations in the low to mid-10+19-cm−3 and optical dielectric function e∞=5.8. The optical properties in the visible and near IR spectral ranges were analyzed by transmission spectroscopy, showing an absorption edge around 1.5eV. The shift of the absorption edge correlates with deviations in the InN stoic...

Reflective second harmonic generation from ZnO thin films: A study on the Zn-O bonding

The structures of the Zn–O bonding in ZnO (0002) thin films prepared by metal organic chemical vapor deposition have been studied by reflective second harmonic generation (RSHG). The polar Zn–O bond on the top layer is not canceled out and presents 3mm symmetrical structures on the well-grown ZnO (0002) surface. The average polar strength of the Zn–O bond is correlated with the quality of the ZnO (0002) thin film. The mirror symmetry is caused by the nonvanished polar of twin boundary due to the mismatch between the ZnO film and sapphire substrate and analyzed using s-polarized RSHG with s-polarized fundamental light irradiation. These results demonstrate that the Zn–O heteropolar bonds on the smooth ZnO surface contribute to the SHG intensity.

Metal organic chemical vapor deposition of crack-free GaN-based light emitting diodes on Si (111) using a thin Al2O3 interlayer

Crack-free GaN light emitting diodes (LEDs) have been grown by metal organic chemical vapor deposition on Si(111) substrates using an atomic layer deposition-grown Al2O3 interlayer. Devices on Si show a longer emission wavelength compared to those on sapphire. This is attributed to tensile strain in the layers on Si, which may increase indium incorporation. Internal quantum efficiency is similar on both substrates. Luminescence intensity versus current density measurements show higher efficiency for the LEDs on Si relative to sapphire at high drive currents. These results show comparable performance characteristics for GaN-based devices on Si and sapphire substrates.

n-type, p-type and semi-insulating ZnO:N thin film growth by metal organic chemical vapor deposition with NH3 doping

p-type, n-type and semi-insulating ZnO:N thin films were successfully grown by metal organic chemical vapor deposition on c-plane sapphire using diethyl zinc and O2 precursors, N2 carrier gas, and NH3 as dopant. NH3 flow rates were varied from 0.2% to 4% in the growth runs. The resulting films were characterized for their structural, optical, and electrical properties by scanning electron microscopy, x-ray diffraction (XRD), Raman spectroscopy, photoluminescence (PL), and Hall effect measurements. XRD show a single ZnO (002) peak; Raman data show the presence of ZnO:N modes at 275, 510, 575, and 645cm−1; and PL results show broad peaks at 480 and 600nm corresponding to deep N incorporation for all the samples. Hall effect show n-type films with carrier concentrations of 6.57×1018cm−3, p-type with carrier concentrations of 4.24×1014cm−3, and semi-insulating with resistivity on the order of 1.5×105Ωcm.

Transition metal and rare earth-doped ZnO: a comparison of optical, magnetic, and structural behavior of bulk and thin films

Recent theoretical predictions of ferromagnetic behavior in transition metal (TM)-doped ZnO have focused significant attention on these materials for use as spintronic materials. Moreover, rare earth (RE) elements in wide bandgap semiconductors would be useful not only in spintronics but also in optoelectronic applications. This work presents results obtained from an investigation into the optical, magnetic, and structural properties of transition-metal (TM)- doped ZnO and rare earth (RE) doped ZnO (TM = Mn, Co, Ni, and Fe; RE = Gd, Eu, and Tb) bulk crystals and thin films. Properties of TM- and RE-doped ZnO bulk crystals and thin films were studied and compared in order to better understand the nature of these dopant centers and their effects on the properties of the host crystal. Optical properties confirm the incorporation of substitutional transition metal ions on cation sites. While most thin film samples show ferromagnetic behavior, the magnetic response of the bulk crystals varies. This suggests that the magnetic behavior of TM-doped ZnO is highly dependent on growth conditions, and growth conditions which favor the formation of grain boundaries and interfaces may be more likely to result in ferromagnetic behavior. Origins of this ferromagnetic behavior are still under investigation. Defect luminescence observed in the RE-doped samples suggests that these materials may prove useful in optoelectonic applications as well.

High indium composition (≫20%) InGaN epi-layers on ZnO substrates for very high efficiency solar cells

In this report we present recent results for MOCVD growth of high indium content InGaN films on ZnO substrates. Growth was attempted on both bulk ZnO as well as ZnO epilayers grown on sapphire by MOCVD. ZnO is an attractive alternative substrate for III-Nitrides because of its superior lattice match: specifically ZnO is perfectly matched with In0.18Ga0.82N and low cost of substrates. Stable InGaN films with ≫18% indium were achieved on the bulk substrates and were characterized by HRXRD, PL, and optical transmission. Varying the growth parameters - primarily growth temperature and In/(In + Ga) flow ratio - was found to affect the optical and structural properties of the films. By growing on a better matched substrate the high indium composition InGaN epitaxial films experience less strain and can therefore be grown thicker without creating relaxation-induced extended crystal defects. This is important, as high indium content InGaN films cannot be grown on GaN thick enough for full above-bandgap absorption without introducing detrimental extended crystal defects. This limitation is thought to be a limiting factor in the achievable ISC in InGaN solar cells.

Manganese-induced long-range lattice disorder and vacancy formation in metal-organic chemical vapor deposition grown and ion-implanted Ga1−xMnxN

The structural properties and lattice dynamics of Ga1−xMnxN were studied for Mn concentrations from 0.0% to 1.5%. Ga1−xMnxN layers were fabricated by either Mn incorporation during the metal-organic chemical vapor deposition (MOCVD) growth process or by postgrowth ion implantation into MOCVD-grown GaN epilayers. The crystalline integrity and the absence of major second phase contributions were confirmed by high-resolution x-ray diffraction analysis. Raman spectroscopy showed that increased Mn incorporation in the epilayers significantly affected long-range lattice ordering, revealing a disorder-induced mode at 300cm−1 and a local vibrational mode at 669cm−1. The low intensities of both modes were shown to scale with Mn concentration. These observations support the formation of nitrogen vacancies, even under optimized MOCVD growth conditions. The slight excess of metal components in the growth process compared to undoped GaN growth and the incorporation of Mn deep acceptor levels favors the formation of ni...

Development of new substrate technologies for GaN LEDs: atomic layer deposition transition layers on silicon and ZnO

Al2O3 layers have been deposited by atomic layer deposition (ALD) on both silicon and zinc oxide (ZnO) substrates as a transition layer for MOCVD growth of GaN. These Al2O3 layers have been shown to reduce tensile strain and cracking in GaN thin films on Si, and they have also been shown to help suppress impurity diffusion from the ZnO substrate into the GaN layers. Surface morphology of the ALD-grown layers was investigated using scanning electron microscopy (SEM), and structural properties were studied using high resolution x-ray diffraction (HR-XRD). GaN thin films were then grown on these layers to determine the effects of the Al2O3 layer on subsequent GaN quality. The optical and structural properties of these films were studied, as well as surface morphology. GaN layers grown using the Al2O3 layers on Si in particular exhibit structural and optical properties approaching those of typical GaN thin films on sapphire, which shows significant promise for high performance GaN-based devices on Si substrates.

Rutherford backscattering and optical studies for ZnO thin films on sapphire substrates grown by metalorganic chemical vapor deposition

A series of ZnO thin films with different thicknesses grown on sapphire substrates by metalorganic chemical vapor deposition (MOCVD) have been studied by different characterization techniques. The optical properties are investigated by photoluminescence (PL), optical transmission (OT) and 1st order derivatives, various angle scanning ellipsometry (VASE). Rutherford Backscattering (RBS) shows the atomic Zn:O ratios with a few percentage aviation from 1:1, and thicknesses in range of 10~230 nm, roughness layer with 10~30nm, which are corresponding to results from atomic force microscopy (AFM), and scanning electron microscopy (SEM). The optical and structure characterization measurements have confirmed the good quality of these epitaxial ZnO materials.

MOCVD growth of GaN on Si substrates using an ALD-grown Al2O3 interlayer

Device-quality GaN thin films have been grown on Si(111) substrates using an Al2O3 transition layer, and initial devices show performance similar to comparable devices on sapphire. X-ray diffraction rocking curve scans show a linewidth of 378 arcsec for the GaN (0002) reflection. Comparison of these layers to GaN layers grown on bare Si substrates shows a significant reduction in strain with the use of the Al2O3 transition layer. Raman spectroscopy measurements verify this reduction in strain, as shown by the shift of the GaN E2(high) with variations in Al2O3 layer thickness. GaN-based devices were also grown and fabricated using this process. Devices on Si showed an IQE of ~32%, which is comparable to the ~37% observed for similar devices on sapphire. The devices on Si also showed better efficiency at high current densities compared to the devices on sapphire, despite the longer peak emission wavelength on Si, which may be due to a difference in thermal conductivity between Si and sapphire. A growth process has been developed for high-quality GaN on Si, and initial device results show that Si is a viable substrate technology for MOCVD growth of GaN-based devices.