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Growth of nonpolar and semi-polar GaN and GaN-based structures offers the opportunity to reduce quantum confined Stark effect and possibly increase indium incorporation, as compared to polar structures, for enhanced performance in green and longer wavelength light emitters. However, the development of the nonpolar and semi-polar GaN growth is hampered by the lack of suitable substrates. Silicon, despite its large thermal-expansion and lattice mismatch with GaN, provides the advantages of the availability of large-size wafers with high crystalline quality at low cost, good electrical conductivity, and feasibility of its removal through chemical etching for better light extraction and heat transfer. In this article, we overview the recent progress in epitaxial growth of nonpolar and semi-polar GaN-based structures on patterned Si substrates. Also discussed are structural and optical properties of the resulting material.

The optical quality of semipolar (1 101)GaN layers was explored by time- and polarization-resolved photoluminescence spectroscopy. High intensity bandedge emission was observed in +c-wing regions of the stripes as a result of better structural quality, while -c-wing regions were found to be of poorer optical quality due to basal plane and prismatic stacking faults (BSFs and PSFs) in addition to a high density of TDs. The high optical quality region formed on the +cwings was evidenced also from the much slower biexponential PL decays (0.22 ns and 1.70 ns) and an order of magnitude smaller amplitude ratio of the fast decay (nonradiative origin) to the slow decay component (radiative origin) compared to the -c-wing regions. In regard to defect-related emission, decay times for the BSF and PSF emission lines at 25 K (~ 0.80 ns and ~ 3.5 ns, respectively) were independent of the excitation density within the range employed (5 – 420 W/cm2), and much longer than that for the donor bound excitons (0.13 ns at 5 W/cm2 and 0.22 ns at 420 W/cm2). It was also found that the emission from BSFs had lower polarization degree (0.22) than that from donor bound excitons (0.35). The diminution of the polarization degree when photogenerated carriers recombine within the BSFs is another indication of the negative effects of stacking faults on the optical quality of the semipolar (1101)GaN. In addition, spatial distribution of defects in semipolar (1101)-oriented InGaN active region layers grown on stripe patterned Si substrates was investigated using near-field scanning optical microscopy. The optical quality of -c- wing regions was found to be worse compared to +c-wing regions due to the presence of higher density of stacking faults and threading dislocations. The emission from the +c-wings was very bright and relatively uniform across the sample, which is indicative of a homogeneous In distribution.

Nonpolar m -plane GaN layers were grown on patterned Si (112) substrates by metal-organic chemical vapor deposition (MOCVD). A two-stage growth procedure involving a low-pressure (30 Torr) first stage to ensure formation of the m -plane facets and a high-pressure stage (200 Torr) for improvement of optical quality was employed. The layers grown in two steps show improvement of the optical quality: the near-bandedge photoluminescence (PL) intensity is about 3 times higher than that for the layers grown at low pressure, and deep emission is considerably weaker. However, emission intensity from m- GaN is still lower than that of polar and semipolar ( 1100 ) reference samples grown under the same conditions. To shed light on this problem, spatial distribution of optical emi ssion over the c+ and ci wings of the nonpolar GaN/Si was studied by spatially resolved cathodoluminescence and near-field scanning optical microscopy. Keywords: GaN, nonpolar, m-plane, Si substrate, MOCVD, time-resolved photoluminescence, near-field scanning optical microscopy

In this work we compare electronic transport performance in HFETs based on single channel (SC) GaN/Al0.30GaN/AlN/GaN (2nm/20nm/1nm/3.5μm) and coupled channel (CC) GaN/Al0.285GaN/AlN/GaN/AlN/GaN (2nm/20nm/1nm/4nm/1nm/3.5μm) structures. The two structures have similar current gain cut-off frequencies (11.6 GHz for SC and 14 GHz for CC for ~ 1μm gate length) however, the maximum drain current, IDmax, is nearly doubled in the CC HFET (0.64 A/mm compared to 0.36 A/mm in SC). HFETs exhibit maximum transconductance (Gmmax) at a bias point close to where maximum f T occurs: VGS =-2.25 V and VDS =12 V and VGS = -2 V and VDS= 15 V for SC and CC HFETs, respectively. Since threshold voltage (Vth) is ~ -3.75 V for both SC and CC structures, devices are able to work at high frequencies with a high gm delivering higher ID. This is in contrast with device performance reported by others where f T is attained at VGS closer to Vth and therefore with lower ID/IDmax ratios and low Gm. Results are consistent in that CC HFET delivers higher IDmax because of the higher electron mobility (μ) and higher carrier density (n) in the channel. As the saturation drain current, IDsat, is attained at electric fields (~40KV/cm) lower than the critical electric field, Ecr , (~ 150KV/cm for GaN ) the higher f T in CC HFETs can be attributed, mainly, to a higher μ, which is in agreement with the Hall measurements. A higher μ in CC HFET is attributed to a shorter hot phonon lifetime.

Semipolar (11macron01) GaN layers and GaN/InGaN LED structures were grown by metal-organic chemical vapor deposition on patterned (001) Si substrates. Optical properties of the semipolar samples were studied by steady-state and time-resolved photoluminescence (PL). Photon energies and intensities of emission lines from steady-state PL as well as carrier decay times from time-resolved PL were correlated with the distributions of extended defects studied by spatially resolved cathodoluminescence and nearfield scanning optical microscopy. Intensity of donor-bound exciton (DX) emission from both coalesced and non-coalesced semipolar layers is comparable to that of state-of-art c-plane GaN template. To gain insight into the contribution from near surface region and deeper portion of the layers to carrier dynamics in polar c-plane and semipolar (11macron01) GaN, time-resolved PL was measured with two different excitation wavelengths of 267 and 353 nm, which provide different excitation depths of about 50 nm and 100 nm, respectively. Time-resolve PL data indicate that the near-surface layer is relatively free from nonradiative centers (point and/or extended defects), while deeper region of the semipolar film (beyond of ~100 nm in depth) is more defective, giving rise to shorter decay times.

GaN-based vertical cavity structures containing bottom AlN/GaN DBRs with top dielectric DBRs on freestanding c-GaN and all dielectric DBRs on GaN on c-sapphire were investigated. Epitaxial lateral overgrowth (ELO) technique allowed the use of both top and bottom all dielectric reflector stacks without substrate removal and the fabrication of the active region containing InGaN multiple quantum wells entirely on the nearly defect-free laterally grown wing regions to avoid nonradiative centers caused by extended and point defects. Compared with the cavity containing hybrid-DBRs on freestanding GaN, the cavity with all dielectric DBRs exhibited quality factors up to 1200 at high optical excitation and an order of magnitude lower stimulated emission threshold density (nearly 5 μJ/cm 2 ). Vertical to lateral growth ratio for ELO could be enhanced up to 5 by increasing the V/III ratio and employment of NH3 modulation, which minimizes the use of dry etching to reduce the cavity thickness and therefore is promising for high quality vertical cavities with all dielectric DBRs.

For high efficiency at high current injection InGaN light emitting diodes (LEDs) necessitate active regions that can mitigate the aggravating electron overflow. Multi double-heterostructures (DHs), 3D active regions separated by low energy barriers, were investigated as optimum solutions for high efficiency as they can accommodate a larger number of states compared to multiple quantum wells (MQWs). However, the number of DH active regions is limited as the material degrades with increasing thickness; therefore, carrier cooling should be partially achieved before the active region using staircase electron injector (SEI) layers. Using electroluminescence (EL) efficiency measurements supported by simulations, active regions and electron injectors were optimized to minimize the electron overflow and the associated efficiency drop at high injection. For a single 3 nm DH LED, the electron overflow was nearly eliminated by increasing the two-step staircase electron injector layer thickness from 4+4 nm to 20+20 nm, whereas the change in SEI thickness had nearly no effect for the DH LEDs with thicker active region. Temperature and excitation density dependent photoluminescence (PL) spectroscopy allowed determination of the material quality and the internal quantum efficiency of device structures with varying active region and SEI thickness.

High resolution transmission electron microscopy and aberration-corrected scanning transmission electron microscopy (STEM) reveal a new void defect in GaN, Si-doped GaN, and InGaN. The voids are pyramid shaped with symmetric hexagonal {0001} base facets and {10-11} side facets. The pyramid void has a closed or open core dislocation at the peak of the pyramid, which continues up along the [0001] growth direction. The closed dislocations have a 1/3 11-20 edge dislocation Burgers vector component, consistent with known threading dislocations. The open core dislocations are hexagonal shaped with pure screw character, {10-10} side facets, varying lateral widths, and varying degrees of hexagonal symmetry. STEM electron energy loss spectroscopy spectrum imaging revealed a larger C concentration inside the void and below the void than above the void. We propose that carbon deposition during metal organic chemical vapor deposition growth acts as a mask, stopping the GaN deposition locally. Subsequent layers of GaN deposited around the C covered region create the overhanging {10-11} facets, and the meeting of the six {10-11} facets at the pyramid's peak is not perfect, resulting in a dislocation.© (2012) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

The effect of active layer design on the efficiency of InGaN light emitting diodes (LEDs) with the light emission in blue (~420 nm) ha s been studied. Correlation between the internal quantum efficiency (IQE) and relative external quantum efficiency (EQE) and salient features of st ructures on c-plane InGaN LEDs which contain multiple quantum wells (MQWs) of different barrier height ( either In 0.01 Ga 0.99 N or In 0.06 Ga 0.94 N barriers ) and thicknes s (3 nm and 12 nm) as well as different double heterostructure (DH) designs (3 nm , dual 3 n m , 6 nm , dual 6 nm , 9 nm and 11 nm) with inserted 3 nm In 0.06 Ga 0.94 N barrier . Pulsed electroluminescence (EL) and optical excitation power -dependent photo luminescence (PL) measurements indic ated that the thinner and lower In 0.06 Ga 0.94 N barriers bode well for high EQE and IQE. Furthermore , increase of the effective active region thickness by multiple InGaN DH structures (dual, quad and hex) separated by 3 nm In

We report on the effects of metal organic epitaxy grown GaN templates with different surface morphologies, achieved under different chamber pressures of 200 and 400 Torr, on the electrical properties of GZO. For as-grown GZO layers with electron concentration above 1020 cm-3 grown on either 200-Torr p-GaN or 400-Torr p-GaN templates, the electron concentration is temperature-dependent as opposed to temperature-independence for GZO/a-sapphires, which demonstrates that the underlying GaN layers affect the GZO electrical properties measured by Hall method. By annealing in nitrogen environment or by inserting a thick ZnO buffer layer, the effects of the underlying GaN layers on GZO electrical properties can be eliminated paving the way for accurate determination of electrical properties. All three annealed GZO layers grown on 200-Torr p-GaN, 400-Torr p-GaN, and a-sapphire, exhibited comparable electron mobilities (~50 cm2/V·s at 15 K and ~41 cm2/V·s at 300 K) and similar temperature dependences while their electron concentrations are different (5.1×1020, 7.1×1020, and 9.2×1020 cm-3) due to the substrate-caused differences in GZO growth mode, structure, etc. By means of simulations, ionized impurity scattering was found to be the dominant scattering mechanism in the range of 15-330 K for GZO when electron concentration is higher than 5×1020 cm-3. Although other scattering events caused by defects and structures are weaker than the ionized impurity scattering, the electrical properties could be still slightly improved by finding more optimized growth conditions to eliminate defects and/or to improve crystal quality.© (2012) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

We apply a number of all-optical time-resolved techniques to study the dynamics of free carriers in InGaN quantum structures under high excitation regime. We demonstrate that carrier lifetime and diffusion coefficient both exhibit a substantial dependence on excitation energy fluence: with increasing carrier density, carrier lifetime drops and diffusivity increases; these effects become more apparent in the samples with higher indium content. We discuss these experimental facts within a model of diffusion-enhanced recombination, which is the result of strong carrier localization in InGaN. The latter model suggests that the rate of non-radiative recombination increases with excitation, which can explain the droop effect in InGaN. We use the ABC rate equation model to fit light induced transient grating (LITG) kinetics and show that that linear carrier lifetime drops with excitation (i.e. excess carrier density). We do not observe any influence of Auger recombination term, CN3, up to the maximum carrier density that is limited due to the onset of very fast stimulated recombination process. To support these conclusions, we present spectrally resolved differential transmission data revealing different recombination rates of carriers in localized and extended states.© (2012) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

Semipolar (1-101) GaN layers were grown by metal-organic chemical vapor deposition on patterned (001) Si substrates. The effects of reactor pressure and substrate temperature on optical properties of (1-101) GaN were studied by steadystate and time-resolved photoluminescence. The optical measurements revealed that the optical quality of (1-101)- oriented GaN is comparable to that of c-plane GaN film grown on sapphire. Slow decay time constants, representative of the radiative recombination, for semipolar (1-101)GaN grown at 200 Torr are found to be very long (~1.8 ns), comparable to those for the state-of-art c-plane GaN templates grown using in situ epitaxial lateral overgrowth through silicon nitride nano-network. Defect distribution in the GaN stripes was studied by spatially resolved cathodeluminescence measurements. The c+-wing regions of the GaN stripes were found to be dominated by a (D0,X) emission. Only a thin slice of emission around 3.42 eV related to basal stacking faults was revealed in c--wing regions.© (2012) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

We report on the effects of substrate temperature and surface morphology of p-GaN templates on the properties of ZnO:Ga (GZO) layers grown by plasma-assisted molecular beam epitaxy. Substrate temperature varying from 200 °C to 450°C was found to have only a moderate effect on the electrical properties of GZO films but it greatly affects the surface morphology of the GZO films. The surface morphology and growth mode of GZO were also found to be considerably affected by the surface morphology of underlying p-GaN templates. On p-GaN templates with a smooth surface (RMS = 0.4 nm) featured by clear atomic steps, GZO layers grew in 2D growth mode and exhibited smooth surfaces with RMS roughness of 2 nm. In contrast, on p-GaN without clear atomic steps but having comparable surface roughness of 0.6 nm, GZO layers grew in 3D growth mode and exhibited rough surface (RMS roughness of ~17.0-20.0 nm). The results of surface roughness are consistent with those from TEM measurements. The lowest resistivity of ~2.3×10 -4 Ω·cm for as-grown GZO layers has been achieved at substrate temperature of 350°C, while the data for 2D GZO layers was affected by a parallel conduction channel from underneath GaN and require further studies. Although the differences in electrical properties and surface morphology existed, the GZO layers grown on different p-GaN templates showed optical transparency higher than 90% in the visible spectral range. The performance of 3D GZO layers as p-electrode was tested in InGaN light emitting diodes.

We have found composition variations along the growth direction within regions of nominally constant indium composition in InGaN light emitting diode structures grown by metal-organic chemical vapor deposition using atomic resolution Z-contrast imaging in a scanning transmission electron microscope (STEM). Within 60 nm thick nominally In 0.01 Ga 0.99 N layers, we found periodic enhancements in the indium concentration into 4 bands separated by 11 nm. Energy dispersive spectroscopy spectrum imaging confirmed that the higher intensity in the high angle annular dark field (HAADF) Z-contrast STEM images was in fact caused by locally higher indium concentration. We observed no lateral indium composition fluctuations.

Non-polar (1-100 ) and semipolar (1-101)GaN layers were grown on (112) and (001) Si substrates, respectively, by metalorganic chemical vapor deposition. In both cases, grooves aligned parallel to the Si direction were formed by anisotropic wet etching to expose vertical {111}Si facets for growth initiation. The effect of growth conditions (substrate temperature, chamber pressure, ammonia and trimethylgallium flow rates) on the growth habits of GaN was studied. It was found that low pressure and low ammonia flow rate are beneficial for m-facet formation, while high ammonia flow rate promotes formation of (1-101) facets. Steady-state and time-resolved photoluminescence measurements revealed that the optical quality of (1-101) oriented GaN is comparable to that of c-plane GaN film grown on sapphire. The nonpolar (1-100 ) GaN shows only weak emission and fast non-radiative recombination rate. The poor optical quality of the mplane GaN can be explained by carbon incorporation during the growth under low pressure. Although further optimization of the growth conditions for better optical quality is required, preliminary results obtained for semipolar (1-101) -oriented GaN are encouraging.

InGaN light-emitting diodes (LEDs) utilizing ZnO layers heavily doped with Ga (GZO) as transparent p -electrodes were fabricated and their characteristics were demonstrated to be superior to those of LEDs with metal Ni/Au electrodes. Highly conductive and highly transparent GZO films were grown on p -GaN contact layers of the LED structures by plasma-assisted molecular beam epitaxy under metal-rich conditions. The c and a lattice constants of GZO were found to be close to the bulk values, indicating small lattice distortion of GZO. The as-grown GZO films showed resistivities as low as 2.2-2.9×10 -4 Ω cm. Upon rapid thermal annealing at the optimum temperature of 675 °C, the resistivity decreased reaching a value of ~1.9×10-4 Ω cm. Unlike the LEDs with Ni/Au contacts, the LEDs with GZO electrodes showed no filamentation and very uniform light emission at high current densities. The peak value of the relative external quantum efficiency for the LEDs with GZO contacts has substantial improvement compared with that for the LEDs with Ni/Au contacts. Under pulsed excitation mode, GZO-LEDs withstood current densities up to 5000 A/cm 2 .

We have studied the effects of thermal annealing in air on photoluminescence of bulk ZnO crystals grown by hydrothermal technique and nominally undoped ZnO layers grown by molecular beam epitaxy on sapphire. Annealing of the samples in air at temperatures above 600°C resulted in a dramatic enhancement of the Cu -related green luminescence (GL) band peaking at 2.45 eV and having characteristic fine structure. The GL band quenched at temperatures above 300 K due to escape of holes from the excited state of the Cu Zn acceptor to the valence band. SIMS profiles revealed moderate increase of Al concentration and significant increase of Cu concentration in annealed samples. Exciton bound to hydrogen-related donor (the 3.363 eV line) quenched after annealing the sample at temperatures above 750oC.

The effect of plasma-induced ion damage on the optical prop erties of ZnO films grown by plasma-assisted molecularbeam epitaxy on a-sapphire substrates and GaN(0001)/ c-sapphire templates prepared has been studied using steady-stateand time-resolved photoluminescence. We observed that the deflecting the ions produced by the RF oxygen plasmaaway from substrate results in improved excitonic emission and modification of the defect-related PL spectrum. Theintensity of the near-band-edge lines in the photoluminescence spectra from the layers grown with the ion deflection was found to increase by factors 7 to 20 for the layers grown on GaN(0001)/ c-sapphire at a plasma power of 350 W and by 3 to 4 times for ZnO grown on a-sapphire substrates at a plasma power of265 W as compared to the controls grownwithout the ion deflection. The yellow-green spectral range was dominated by different defect bands in the films grownwith and without ion deflectio n. The effect of RF power on peak positions of the defect band was studied for the filmsgrown without ion deflection. For the ZnO films grown on a-plane sapphire substrates, time-resolved photoluminescenceshowed a significant increase in luminescence decay times both at RT and 89 K. However, for ZnO on GaN(0001)/c-sapphire substrates, virtually no improvement in decay time was found at 89 K with only a moderate increase in decayconstant at room temperature.Keywords: experiment, ZnO, molecular beam epitaxy, photoluminescence

Epitaxial growth of ZrO 2 has been achieved on MOCVD-grown GaN(0001) templates by oxides molecular beam epitaxy using reactive H 2 O 2 for oxygen and organometallic source for Zr. Utilizing a low temperature buffer layer followed by high temperature insitu annealing and high-temperature growth, monoclinic (100)-oriented ZrO 2 thin films were obtained. The full width at half maximum of ZrO 2 (100) rocking curve was 0.4 arc degree for 30-nm-thick films and the rms roughness for a 5mm by 5 mm AFM scan was 4 a. The employment of epitaxial ZrO 2 layer in the AlGaN/GaN heterojunction field effect trasnsistor as a gate dielectric has resulted in the increase of the saturation-current density and pinch-off voltage as well as in near symmetrical gate-drain I-V behavior.

ZnO nanorods were grown by catalyst-assisted vapor phase transport on Si(001), GaN(0001)/c-Al2O3, and bulk ZnO(0001) substrates. Morphology studies as well as X-ray diffraction and transmission electron microscopy showed that ZnO nanorods grew mostly perpendicular to the GaN(0001) and ZnO(0001) substrate surface, whereas a more random directional distribution was found for nanorods on Si(001). Integral optical properties of fabricated nanorods were studied by steady-state photoluminescence and time-resolved photoluminescence. Stimulated emission was observed from ZnO nanorods on GaN(0001)/c-Al2O3 substrates, most likely due to their vertical orientation. Near-field scanning optical microscopy was applied to investigate luminescent properties of individual rods. Raman spectroscopy revealed biaxial compressive strain in the nanorod samples grown on Si(001). Conductive atomic force microscopy showed that nanorods are electrically isolated from each other. I-V spectra of individual nanorods were measured.

Due to their unique physical properties GaN-based heterostructures show a great promise for spintronics applications. This stimulates the search for GaN-based ferromagnetic semiconductors which can be used for injection of spin polarized carriers in device structures. In this study, magnetic properties of GaN layers implanted with Gd + ions to various doses were investigated. Magnetization curves of samples with Gd content n Gd = 2x10 17 and 2x10 18 cm -3 show clear hysteresis, while the samples with n Gd = 2x10 16 and 2x10 19 cm -3 exhibit no ferromagnetism. Most likely, the lowest Gd concentration produced magnetization below the detection limit, whereas the absence of ferromagnetism in the sample with the highest Gd content may be resulted from heavy implantation-induced damage. Curie temperatures for samples with Gd contents of 2x10 17 and 2x10 18 cm -3 were estimated to be larger than 300 K. Saturation magnetizations of 1550 μ B and 1350 μ B per Gd-atom were found at 5 K and 300 K, respectively, for the sample with n Gd =2x10 18 cm -3 .

Optical properties of ZnO doped with Mn and V were studied. Zn(Mn)O layers were grown by peroxide MBE, and Zn(V)O was prepared by high-dose ion implantation of bulk ZnO prepared by hydrothermal technique. The Zn(Mn)O layers containing up to 50% of Mn were characterized by high-resolution x-ray diffraction, photoluminescence, and optical absorption. A blue shift of the band edge revealed from optical absorption measurements points to the incorporation of at least a part of Mn atoms on the lattice sites. An increase in the Zn(Mn)O band gap and an enhancement of the broad below band gap absorption associated with Mn ions were observed with increasing Mn composition. Correlating structural and optical transmission data, we suggest that the band edge of Zn(Mn)O rises linearly with the amount of Mn ions substituting Zn on the lattice sites. Photoluminescence of ZnO moderately doped with Mn shows several emission lines (the strongest ones are located at 3.34 and 3.36 eV). Surprisingly, no shift in the near-band-edge emission (3.36 eV) was detected in the photoluminescence data. Photoluminescence excitation studies revealed that the near-band-edge peak and the peak centered around 3.34 eV have different origin. Most probably, the second line is due to Mn intracenter transitions. Photoluminescence studies of ZnO bulk samples implanted with V + have revealed that thermal annealing at 800 °C restores to a large extent the optical quality of the material. A new emission line centered at 3.307 eV has been found in the photoluminescence spectrum of the highly conductive samples implanted with a V dose of 1 × 10 16 cm -2 .

Ferromagnetic Ga1-xMnxAs films containing up to 5.1 at%Mn were grown by low-temperature MBE. The structural, electrical, and magnetic properties of the layers are reported. At x > 0.01, the materials show a ferromagnetic behavior. The Curie temperature reaches 80 K at 5.1at% Mn. We propose the use of a n+-GaAs/p+-GaMnAs Esaki-diode (ferromagnetic Esaki-diode, FED) to provide injection of spin-polarized electrons via interband tunneling. Under reverse bias, spin-polarized electrons at the Fermi level in the valence band of GaMnAs tunnel to the conduction band of GaAs in contrast to the injection of spin-polarized holes used before.© (2002) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.