Pressure Metalorganic Chemical Vapor Research Articles

Optically-pumped 285 nm edge stimulated emission from AlGaN-based LED structures grown by MOCVD on sapphire substrates

285 nm stimulated emission (SE) is demonstrated at room temperature from optically-pumped AlGaN multiple quantum wells (MQWs) deposited between a Si-doped AlGaN current spreading layer and a 25 nm Mg-doped p-type AlGaN clad layer. The epitaxial structures were grown by low pressure metalorganic chemical vapor deposition (MOCVD) on high-quality AlN/sapphire templates and a pulsed ArF excimer laser was used as the excitation source for lasing experiments. The threshold power density (Pth) was measured to be 970 kW/cm2 and the SE was strongly polarized in transverse electric (TE) mode. The minimum emission linewidth was ∼2 nm. The high value of Pth is primarily attributed to optical losses due to pump laser absorption in the top p-AlGaN. Our results show the viability of the MOCVD growth technology in conjunction with c-plane sapphire substrates to develop electrically-pumped deep-UV laser diodes.

Efficiency improvement of GaN-based ultraviolet light-emitting diodes with reactive plasma deposited AlN nucleation layer on patterned sapphire substrate.

The flip chip ultraviolet light-emitting diodes (FC UV-LEDs) with a wavelength of 365 nm are developed with the ex situ reactive plasma deposited (RPD) AlN nucleation layer on patterned sapphire substrate (PSS) by an atmospheric pressure metal-organic chemical vapor deposition (AP MOCVD). The ex situ RPD AlN nucleation layer can significantly reduce dislocation density and thus improve the crystal quality of the GaN epitaxial layers. Utilizing high-resolution X-ray diffraction, the full width at half maximum of the rocking curve shows that the crystalline quality of the epitaxial layer with the (RPD) AlN nucleation layer is better than that with the low-temperature GaN (LT-GaN) nucleation layer. The threading dislocation density (TDD) is estimated by transmission electron microscopy (TEM), which shows the reduction from 6.8 × 107 cm−2 to 2.6 × 107 cm−2. Furthermore, the light output power (LOP) of the LEDs with the RPD AlN nucleation layer has been improved up to 30 % at a forward current of 350 mA compared to that of the LEDs grown on PSS with conventional LT-GaN nucleation layer.

Performance enhancement of GaN-based flip-chip ultraviolet light-emitting diodes with a RPD AlN nucleation layer on patterned sapphire substrate

In this work, flip-chip ultraviolet light-emitting diodes (FCUV-LEDs) on patterned sapphire substrate (PSS) at 375 nm were grown by an atmospheric pressure metal organic chemical vapor deposition (AP-MOCVD). A specialized reactive plasma deposited (RPD) AlN nucleation layer was utilized on the PSS to enhance the quality of the epitaxial layer. By using high-resolution X-ray diffraction, the full-width at half-maximum of the rocking curve shows that the FCUV-LEDs with RPD AlN nucleation layer had better crystalline quality when compared to conventional GaN nucleation samples. From the transmission electron microscopy (TEM) image, it can be observed that the tip and incline portion of the pattern was smooth using the RPD AlN nucleation layer. The threading dislocation densities (TDDs) are reduced from 7 × 107 cm−2 to 2.5 × 107 cm−2 at the interface between the u-GaN layers for conventional and AlN PSS devices, respectively. As a result, a much higher light output power was achieved. The improvement of light output power at an injection current of 20 mA was enhanced by 30%. Further photoluminescence measurement and numerical simulation confirm such increase of output power can be attributed to the improvement of material quality and light extraction.

High performance 380-nm ultraviolet light-emitting-diodes with 3% efficiency droop by using free-standing GaN substrate manufacturing from GaAs substrate

We investigated the influence of free-standing GaN (FS-GaN) substrates on the performance of ultraviolet light-emitting-diodes (UV-LEDs) grown on top by atmospheric pressure metal-organic chemical vapor deposition. High-resolution double-crystal x-ray diffraction (HRDCXD) analysis demonstrated high-order satellite peaks and clear fringes between them for UV-LEDs grown on the FS-GaN substrate, from which the interface roughness was estimated. In addition, the full width at half maximum of the HRDCXD rocking curve in the (0002) and the (101¯2) reflections were reduced to below 90 arc sec. The Raman results indicated that the GaN-based epilayer of strain free was grown. Additionally, the effect of the FS-GaN substrate on the crystal quality of the UV-LEDs was examined in detail by transmission electron microscopy (TEM). The TEM characterizations revealed no defects and V-pits were found in the scanned area. Based on the results mentioned above, the light output power of UV-LEDs on the FS-GaN substrate can be enhanced drastically by 80% and 90% at 20 and 100 mA, respectively. Furthermore, an ultralow efficiency degradation of about 3% can be obtained for the UV-LEDs on the FS-GaN substrate at a high injection current. The use of an FS-GaN substrate is suggested to be effective for improving the emission efficiency and droop of UV-LEDs grown thereon.

Nucleation of GaSb on GaAs (001) by low pressure metal—organic chemical vapor deposition

The initial growth stage of GaSb on GaAs (001) by low pressure metal—organic chemical vapor deposition (MOCVD) is investigated. The dependence of the nucleation on growth temperature, growth pressure, and vapor V/III ratio is studied by means of atomic force microscopy. The nucleation characteristics include the island density, size, and size uniformity distribution. The nucleation mechanism is discussed by the effects of growth temperature, growth pressure, and vapor V/III ratio on the density, size, and size uniformity of GaSb islands. With the growth temperature increasing from 500 °C to 610 °C and the growth pressure increasing from 50 mbar to 1000 mbar (1 mbar = 105 Pa), the island density first increases and then decreases; with the V/III ratio increasing from 0.5 to 3, the trend is contrary.

High power 940 nm quantum well laser with asymmetric structure

In order to improve the performance of the general broad area high power 940 nm InGaAs/GaAs semiconductor laser diode (LD), we design a new type quantum well LD with an asymmetric waveguide and cladding structure. High material quality with low internal loss of 0.44 mm-1 has been achieved using low pressure metal organic chemical vapor deposition (LP-MOCVD) method. Broad-area lasers were fabricated with the wafers grown on GaAs substrates. For the 940 nm devices with 100 m-wide stripe and 2000 m-long cavity under 25 ℃ continuous wave (CW) operation condition of 10 A, the typical threshold current is 251 mA, the slope efficiency is 1.22 W/A , and the maximum output power reaches 9.6 W. The laser diode yielded a maximum power conversion efficiency over 70%.

Inline atmospheric pressure metal-organic chemical vapour deposition for thin film CdTe solar cells

A detailed study has been undertaken to assess the deposition of CdTe for thin film devices via an inline atmospheric pressure metal-organic chemical vapour deposition (AP-MOCVD) reactor. The precursors for CdTe synthesis were released from a showerhead assembly normal to a transparent conductive oxide (TCO)/glass substrate, previously coated with a CdZnS window layer using a conventional batch AP-MOCVD reactor with horizontal flow delivery. Under a simulated illumination with air mass coefficient 1.5 (AM1.5), the initial best cell conversion efficiency (11.2%) for such hybrid cells was comparable to a reference device efficiency (~13%), grown entirely in the AP-MOCVD batch reactor. The performance and structure of the hybrid and conventional devices are compared for spectral response, CdTe grain morphology and crystal structure. These preliminary results reported on the transfer from a batch to an inline AP-MOCVD reactor which holds a good potential for the large-scale production of thin film photovoltaics devices and related materials.

Effect of band alignment on photoluminescence and carrier escape from InP surface quantum dots grown by metalorganic chemical vapor deposition on Si

A detailed analysis of photoluminescence (PL) from InP quantum dots (QDs) grown on Si has been carried out to understand the effect of substrate/host material in the luminescence and carrier escape process from the surface quantum dots. Such studies are required for the development of monolithically integrated next generation III-V QD based optoelectronics with fully developed Si microelectronics. The samples were grown by atmospheric pressure metalorganic chemical vapor deposition technique, and the PL measurements were made in the temperature range 10–80 K. The distribution of the dot diameter as well as the dot height has been investigated from atomic force microscopy. The origin of the photoluminescence has been explained theoretically. The band alignment of InP/Si heterostructure has been determined, and it is found be type II in nature. The positions of the conduction band minimum of Si and the 1st excited state in the conduction band of InP QDs have been estimated to understand the carrier escape phenomenon. A blue shift with a temperature co-efficient of 0.19 meV/K of the PL emission peak has been found as a result of competitive effect of different physical processes like quantum confinement, strain, and surface states. The corresponding effect of blue shift by quantum confinement and strain as well as the red shift by the surface states in the PL peaks has been studied. The origin of the luminescence in this heterojunction is found to be due to the recombination of free excitons, bound excitons, and a transition from the 1st electron excited state in the conduction band (e1) to the heavy hole band (hh1). Monotonic decrease in the PL intensity due to increase of thermally escaped carriers with temperature has been observed. The change in barrier height by the photogenerated electric-field enhanced the capture of the carriers by the surface states rather than their accumulation in the QD excited state. From an analysis of the dependence of the PL intensity, peak position, and line width with temperature and excitation source, the existence of free and bound excitonic recombination together with e1 → hh1 transitions in the QDs is established.

Structure and composition analysis of the Cu–Zn–Se ternary compounds by TEM/EDS

The structure and chemical composition of Cu1−xZn1−ySe2−δ (Cu–Zn–Se) ternary alloy semiconductors were examined by transmission electron microscopy (TEM) coupled with energy dispersive X-ray spectroscopy (EDS), a TEM/EDS analytical technique. Polycrystalline Cu–Zn–Se thin films were prepared by atmospheric pressure metal–organic chemical vapor deposition. The TEM/EDS technique has made an elemental analysis focused on a microscopic region possible. Therefore, the spatial distribution of constituent elements of Cu–Zn–Se polycrystal within both grains and grain boundaries could be investigated and the compound stoichiometry was evaluated. Conclusively, the cross-sectional TEM images characterized the fabricated Cu–Zn–Se layers as a polycrystalline film of about 70–80nm thick and composed of grains with sizes ranging from 0.3 to 1.7μm. The chalcopyrite structure of Cu–Zn–Se films was confirmed by the selected area electron diffraction patterns. Moreover, the EDS point analyses conducted at optionally chosen positions showed quite comparable chemical compositions for the grains examined, yielding an average composition ratio of about 0.92:1.44:2 for Cu:Zn:(Se+O) within grains. However, the replacement of Cu atoms for Zn atoms in elemental composition was found at grain boundaries, resulting in an average composition ratio of about 1.26:1.24:2 for Cu:Zn:(Se+O) over there. It is suggested that the enhanced diffusion of Cu along the grain boundaries and the segregation of Cu to the grain boundaries increase the mean composition of Cu at these paths.

Effects of TMSb overpressure on InSb surface morphology for InSb epitaxial growth using low pressure metalorganic chemical vapor deposition

We investigated the effects of antimony (Sb) overpressure during thermal cleaning (TC) on the surface morphology of InSb substrates using low pressure metalorganic chemical vapor deposition (LP-MOCVD). During the TC process under H2 ambient, indium (In) droplets were observed inside etch pits on InSb(001) surfaces due to the Sb evaporation from InSb substrates. On the other hand, when InSb substrates were thermally cleaned under trimethylantimony (TMSb) ambient, the formation of In droplets and the etch pits were suppressed, resulting in the smooth InSb surface. The surface morphology of InSb was dependent on TC temperature. Rough surface morphology was observed at low TC temperature of 435°C and it became smoother with increasing TC temperature. The improvement of surface morphology was caused by the surface stabilization with increasing Sb flux and the increase of adatom migration. The dependence of TMSb flow rate on the surface morphology was also investigated. The TMSb overpressure during the TC of InSb must be maintained to grow high quality InSb epitaxial layers with smooth surface using MOCVD.

Fabrication of Cu3Zn3Se ternary compounds by AP-MOCVD

I–II–VI Ternary chalcopyrite semiconductors of Cu1−xZn1−ySe2−δ (Cu3Zn3Se) were successfully fabricated by the atmospheric pressure metal-organic chemical vapor deposition method for the first time. Four major peaks of (112), (220)/(204), (312)/(116), and (400), accompanied by three minor peaks of (103), (211) and (301) were observable in the X-ray diffraction spectra. In particular, the presence of those latter low-intensity peaks featured the formation of the chalcopyrite type Cu3Zn3Se compound. Typical absorption coefficients of the films produced were found to vary from 2.75×104cm−1 to 5.75×104cm−1 over all visible light range and from the same optical absorption spectra the energy gap of the material was extracted to be about 2.02eV. Moreover, the photoluminescence measurement conducted at room temperature also exhibited a strong orange-colored emission line at 1.94eV, which further confirmed that the Cu3Zn3Se ternary compound has been prepared. In addition, the cross-sectional and top-view scanning electron microscopy images characterized the fabricated Cu3Zn3Se compound as a polycrystalline film of thickness about 1–1.3μm and consisting of 1–2.5μm sized grains.

Study of the thermal stability of the H-related donors in high resistivity ZnO:Cu thin films by high-pressure H2 treatment

High resistivity ZnO:Cu thin films were fabricated on c-plane sapphire substrate by low pressure metal-organic chemical vapor deposition (MOCVD). The samples were treated in H2 ambient at high pressure and the thermal stability of the hydrogen-related donor-like defects was studied. Hall-effect measurement results presented that the high resistivity ZnO:Cu samples turn into n-type conductivity after H incorporation. The annealing results indicated that the hydrogenated ZnO:Cu could not recover high resistivity even after annealed at 750°C. It is proposed that some unknown H-related donor-like defects still exist in the film after annealing.

Effect of Silicon Doping on GaN Nanorods during Vapor–Liquid–Solid Growth

High optical quality and reproducible Si-doped GaN nanorods (NRs) were grown by low pressure metal–organic chemical vapor deposition. Different fluxes of SiH4 were introduced and at 2.7×10-9 mol/min the optimized morphology was obtained. Clear improvement of GaN NRs morphology characterized by smooth side walls and straightness was achieved after Si-doping. Near-band edge (NBE) emission at 356.88 nm (3.474 eV) and small full width at half maximum (FWHM) of 27.3 meV, measured by room temperature photoluminescence, confirmed the high optical and crystalline qualities. For Si-doped GaN NRs, a small blue-shift of NBE emission peak was observed, which was attributed to the band-filling effects.

Identification of LiNbO3, LiNb3O8 and Li3NbO4 phases in thin films synthesized with different deposition techniques by means of XRD and Raman spectroscopy

Phase composition of epitaxial/textured LiNbO3 films on sapphire substrates, grown by pulsed laser deposition, atmospheric pressure metal organic chemical vapor deposition and pulsed injection metal organic chemical vapor deposition was studied by conventional x-ray diffraction techniques. Raman spectroscopy, being highly sensitive to the symmetry of materials, was used as a countercheck in the compositional analysis. The wavenumbers of Raman modes of LiNb3O8 and Li3NbO4 phases were identified from Raman spectra of synthesized powders. Asymmetry of profiles of x-ray diffraction reflections of LiNbO3 films was studied. This asymmetry may have different origins which consequently may result in misleading conclusions about phase composition of textured LiNbO3 films.

Growth of ZnO Films on Si(111) by Metalorganic Chemical Vapor Deposition with AlN and Low-Temperature ZnO Double Buffers

To enhance the quality of ZnO films on Si(111) substrate, single layers of low-temperature ZnO (LT-ZnO) and AlN, as well as a combination of AlN and LT-ZnO layer, were used as intermediate layers by atmospheric pressure metalorganic chemical vapor deposition system. Only polycrystalline ZnO film was formed when a LT-ZnO single buffer was used. Crystal quality was enhanced when LT-ZnO was replaced by 20 nm AlN as the single buffer. The full width at half maximum (FWHM) of ZnO(0002) x-ray diffraction ω-rocking curve was 642 arcsec. Cracks began to appear on the film surface as crystallinity was enhanced. A ZnO mosaic single-crystal film with a mirror-like surface was successfully fabricated when a combined AlN and LT-ZnO served as buffer, and its FWHM of ZnO(0002) ω-rocking curve peak was only 460 arcsec. The film surface was smoother but cracks were still evident on the film. Contrary to the three-dimensional growth mode of samples with a single buffer, a quasi-two-dimensional growth mode was realized for the double-buffered high-temperature ZnO layer. Calculated film thickness was 2.14 μm, and the growth rate reached 4.3 μm/h based on the laser in situ laser reflectance trace.

Developing Monolithically Integrated CdTe Devices Deposited by AP-MOCVD

ABSTRACTThin film deposition process and integrated scribing technologies are key to forming large area Cadmium Telluride (CdTe) modules. In this paper, baseline Cd1-xZnxS/CdTe solar cells were deposited by atmospheric-pressure metal organic chemical vapor deposition (AP-MOCVD) onto commercially available ITO coated boro-aluminosilicate glass substrates. Thermally evaporated gold contacts were compared with a screen printed stack of carbon/silver back contacts in order to move towards large area modules. P2 laser scribing parameters have been reported along with a comparison of mechanical and laser scribing process for the scribe lines, using a UV Nd:YAG laser at 355 nm and 532 nm fiber laser.

Effect of O2/Ni ratio on structure and surface morphology of atmospheric pressure MOCVD grown NiO thin films

ABSTRACTEffect of oxygen to nickel molar ratio (O2/Ni) on the crystallinity of atmospheric pressure metal organic chemical vapor deposition (APMOCVD) grown NiO at 500°C is reported. X-ray diffraction (XRD) analysis including grazing incident angle θ of 0.6°, θ-2θ, ɸ and rocking curve scan are employed for crystallographic characterization. Furthermore, surface roughness is studied by atomic force microscopy (AFM). No evidence of diffraction peaks in X-ray grazing incident angle measurement confirms that all the grown NiO films are well oriented along a certain direction. θ-2θ scan results further indicate that the samples are highly oriented only along [111] direction on (0001) sapphire substrates. The analysis of full width at half maximum (FWHM) of rocking curve scan of (111) plane shows that higher O2/Ni ratio results in better crystallinity. The best crystallinity is achieved with FWHM as low as 0.106° at (111) rocking curve scan corresponding to 82.57nm grain size. AFM measurement shows that NiO films grown with higher O2/Ni ratio have smoother surface morphology.

Non-stoichiometric AlOxFilms Prepared by Chemical Vapor Deposition Using Dimethylaluminum Isopropoxide as Single Precursor and Their Non-volatile Memory Characteristics

Dimethylaluminum isopropoxide (DMAI, <TEX>$(CH_3)_2AlO^iPr$</TEX>) as a single precursor, which contains one aluminum and one oxygen atom, has been adopted to deposit non-stoichiometric aluminum oxide (<TEX>$AlO_x$</TEX>) films by low pressure metal organic chemical vapor deposition without an additional oxygen source. The atomic concentration of Al and O in the deposited <TEX>$AlO_x$</TEX> film was measured to be Al:O = ~1:1.1 and any serious interfacial oxide layer between the film and Si substrate was not observed. Gaseous by-products monitored by quadruple mass spectrometry show that <TEX>${\beta}$</TEX>-hydrogen elimination mechanism is mainly contributed to the <TEX>$AlO_x$</TEX> CVD process of DMAI precursor. The current-voltage characteristics of the <TEX>$AlO_x$</TEX> film in Au/<TEX>$AlO_x$</TEX>/Ir metalinsulator-metal (MIM) capacitor structure show high ON/OFF ratio larger than <TEX>${\sim}10^6$</TEX> with SET and RESET voltages of 2.7 and 0.8 V, respectively. Impedance spectra indicate that the switching and memory phenomena are based on the bulk-based origins, presumably the formation and rupture of filaments.

Reflection absorption infrared spectroscopy analysis of the evolution of ErSb on InSb

We discuss an ex-situ monitoring technique based on glancing-angle infrared-absorption used to determine small amounts of erbium antimonide (ErSb) deposited on an indium antimonide (InSb) layer epitaxially grown on an InSb (100) substrate by low pressure metal organic chemical vapor deposition (MOCVD). Infrared absorption from the indium–hydrogen (InH) stretching mode at 1754.5cm−1 associated with a top most surface of an epitaxial InSb layer was used to compare varying levels of surface coverage with ErSb. Among four samples of varying coverage of ErSb deposition (7.2 to 21.5 monolayers), detected infrared absorption peaks distinct to InH weakened as ErSb surface coverage increased. In the early stage of ErSb deposition, our study suggests that outermost indium atoms in the InSb buffer layer are replaced by Er resulting in increase in absorption associated with the InH mode. Using this simple ex-situ technique, we show that it is possible to calibrate the amount of ErSb deposited atop each individual InSb substrate for depositions of few to tens of monolayers.

Influence of the GaN capping thickness on the strain and photoluminescence properties of InN/GaN quantum dots

InN quantum dots (QDs) were grown on 1μm thick GaN/(0001) sapphire substrates by low pressure metal organic chemical vapor deposition. A single crystalline 10-nm thick GaN capping layer was achieved on the InN QDs by the flow-rate modulation epitaxy method at 650°C. The (002) ω/2θ scans of the X-ray diffraction measurements show that the reduction of the lattice constant with a capping thickness indicate that the GaN capping process exerts a compressive strain on the InN QDs. The residual strain was reduced from 0.245% to −0.245% as the GaN cap thickness increases from 0 to 20nm. In addition, the analysis of the photoluminescence peak energy estimated that the free electron concentration (i.e. density of indium (In) vacancy) decreased from 1.62×1018cm−3 to 1.24×1018cm−3. The suggestion here is that the increase of the compressive strain on InN QDs due to the increased GaN capping layer thickness provides the high driving force for the interdiffusion of the In atom and gallium (Ga) atoms between the interface of InN QDs and the GaN capping layer. Thus, we believe that more Ga atoms can diffuse from the GaN capping layer and substitute the high density of In vacancy in the InN QDs, resulting in a decrease of the free electron concentration in the InN QDs with the increase in the GaN capping layer thickness.