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Measurement and thermal modeling of sapphire substrate temperature at III-Nitride MOVPE conditions

Journal of Crystal Growth

Creighton, J.R.; Coltrin, Michael E.; Figiel, J.J.

Growth rates and alloy composition of AlGaN grown by MOVPE is often very temperature dependent due to the presence of gas-phase parasitic chemical processes. These processes make wafer temperature measurement highly important, but in fact such measurements are very difficult because of substrate transparency in the near-IR (~900 nm) where conventional pyrometers detect radiation. The transparency problem can be solved by using a mid-IR pyrometer operating at a wavelength (~7500 nm) where sapphire is opaque. We employ a mid-IR pyrometer to measure the sapphire wafer temperature and simultaneously a near-IR pyrometer to measure wafer pocket temperature, while varying reactor pressure in both a N2 and H2 ambient. Near 1300 °C, as the reactor pressure is lowered from 300 Torr to 10 Torr the wafer temperature drops dramatically, and the ∆T between the pocket and wafer increases from ~20 °C to ~250 °C. Without the mid-IR pyrometer the large wafer temperature change with pressure would not have been noted. In order to explain this behavior we have developed a quasi-2D thermal model that includes a proper accounting of the pressure-dependent thermal contact resistance, and also accounts for sapphire optical transmission. The model and experimental results demonstrate that at most growth conditions the majority of the heat is transported from the wafer pocket to the wafer via gas conduction, in the free molecular flow limit. In this limit gas conductivity is independent of gap size but first order in pressure, and can quantitatively explain results from 20 to 300 Torr. Further analysis yields a measure of the thermal accommodation coefficients; α(H2) =0.23, α(N2) =0.50, which are in the range typically measured.

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TYPE Journal Article YEAR 2017
ScopusOSTIDOI

Nonpolar InGaN/GaN Core-Shell Single Nanowire Lasers

Nano Letters

Li, Changyi; Wright, Jeremy B.; Liu, Sheng L.; Lu, Ping L.; Figiel, J.J.; Leung, Benjamin; Chow, Weng W.; Brener, Igal B.; Koleske, Daniel K.; Luk, Ting S.; Feezell, Daniel F.; Brueck, S.R.J.; Wang, George T.

We report lasing from nonpolar p-i-n InGaN/GaN multi-quantum well core-shell single-nanowire lasers by optical pumping at room temperature. The nanowire lasers were fabricated using a hybrid approach consisting of a top-down two-step etch process followed by a bottom-up regrowth process, enabling precise geometrical control and high material gain and optical confinement. The modal gain spectra and the gain curves of the core-shell nanowire lasers were measured using micro-photoluminescence and analyzed using the Hakki-Paoli method. Significantly lower lasing thresholds due to high optical gain were measured compared to previously reported semipolar InGaN/GaN core-shell nanowires, despite significantly shorter cavity lengths and reduced active region volume. Mode simulations show that due to the core-shell architecture, annular-shaped modes have higher optical confinement than solid transverse modes. The results show the viability of this p-i-n nonpolar core-shell nanowire architecture, previously investigated for next-generation light-emitting diodes, as low-threshold, coherent UV-visible nanoscale light emitters, and open a route toward monolithic, integrable, electrically injected single-nanowire lasers operating at room temperature.

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TYPE Journal Article YEAR 2017
ScopusOSTIDOI

Intrinsic polarization control in rectangular GaN nanowire lasers

Nanoscale

Li, Changyi; Liu, Sheng L.; Luk, Ting S.; Figiel, J.J.; Brener, Igal B.; Brueck, S.R.J.; Wang, George T.

We demonstrate intrinsic, linearly polarized lasing from single GaN nanowires using cross-sectional shape control. A two-step top-down fabrication approach was employed to create straight nanowires with controllable rectangular cross-sections. A clear lasing threshold of 444 kW cm-2 and a narrow spectral line width of 0.16 nm were observed under optical pumping at room temperature, indicating the onset of lasing. The polarization was along the short dimension (y-direction) of the nanowire due to the higher transverse confinement factors for y-polarized transverse modes resulting from the rectangular nanowire cross-section. The results show that cross-sectioned shape control can enable inherent control over the polarization of nanowire lasers without additional environment requirements, such as placement onto lossy substrates.

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TYPE Journal Article YEAR 2016
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Electrically Injected UV-Visible Nanowire Lasers

Wang, George T.; Li, Changyi L.; Li, Qiming L.; Liu, Sheng L.; Wright, Jeremy B.; Brener, Igal B.; Luk, Ting S.; Chow, Weng W.; Leung, Benjamin L.; Figiel, J.J.; Koleske, Daniel K.; Lu, Tzu-Ming L.

There is strong interest in minimizing the volume of lasers to enable ultracompact, low-power, coherent light sources. Nanowires represent an ideal candidate for such nanolasers as stand-alone optical cavities and gain media, and optically pumped nanowire lasing has been demonstrated in several semiconductor systems. Electrically injected nanowire lasers are needed to realize actual working devices but have been elusive due to limitations of current methods to address the requirement for nanowire device heterostructures with high material quality, controlled doping and geometry, low optical loss, and efficient carrier injection. In this project we proposed to demonstrate electrically injected single nanowire lasers emitting in the important UV to visible wavelengths. Our approach to simultaneously address these challenges is based on high quality III-nitride nanowire device heterostructures with precisely controlled geometries and strong gain and mode confinement to minimize lasing thresholds, enabled by a unique top-down nanowire fabrication technique.

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TYPE SAND Report YEAR 2015
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GaN nanowires with pentagon shape cross-section by ammonia-source molecular beam epitaxy

Journal of Crystal Growth

Lin, Yong; Leung, Benjamin L.; Li, Qiming; Figiel, J.J.; Wang, George T.

Ammonia-based molecular beam epitaxy (NH3-MBE) was used to grow catalyst-assisted GaN nanowires on (11¯02) r-plane sapphire substrates. Dislocation free [112¯0] oriented nanowires are formed with pentagon shape cross-section, instead of the usual triangular shape facet configuration. Specifically, the cross-section is the result of the additional two nonpolar {101¯0} side facets, which appear due to a decrease in relative growth rate of the {101¯0} facets to the {101¯1} and {101¯1} facets under the growth regime in NH3-MBE. Compared to GaN nanowires grown by Ni-catalyzed metal-organic chemical vapor deposition, the NH3-MBE grown GaN nanowires show more than an order of magnitude increase in band-edge to yellow luminescence intensity ratio, as measured by cathodoluminescence, indicating improved microstructural and optical properties.

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TYPE Journal Article YEAR 2015
ScopusOSTIDOI

Continuous and dynamic spectral tuning of single nanowire lasers with subnanometer resolution using hydrostatic pressure

Nanoscale

Liu, Sheng L.; Li, Changyi; Figiel, J.J.; Brueck, Steven R.J.; Brener, Igal B.; Wang, George T.

We report continuous, dynamic, reversible, and widely tunable lasing from 367 to 337 nm from single GaN nanowires (NWs) by applying hydrostatic pressure up to ∼7 GPa. The GaN NW lasers, with heights of 4-5 μm and diameters ∼140 nm, are fabricated using a lithographically defined two-step top-down technique. The wavelength tuning is caused by an increasing Γ direct bandgap of GaN with increasing pressure and is precisely controllable to subnanometer resolution. The observed pressure coefficients of the NWs are ∼40% larger compared with GaN microstructures fabricated from the same material or from reported bulk GaN values, revealing a nanoscale-related effect that significantly enhances the tuning range using this approach. This approach can be generally applied to other semiconductor NW lasers to potentially achieve full spectral coverage from the UV to IR.

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TYPE Journal Article YEAR 2015
ScopusOSTIDOI

Nonpolar InGaN/GaN multi-quantum-well core-shell nanowire lasers

CLEO: Science and Innovations, CLEO-SI 2015

Li, Changyi; Wright, Jeremy B.; Liu, Sheng L.; Lu, Ping L.; Figiel, J.J.; Leung, Benjamin L.; Luk, Ting S.; Brener, Igal B.; Feezell, Daniel; Brueck, S.R.J.; Wang, George T.

Lasing is demonstrated from nonpolar III-nitride core-shell multi-quantum-well nanowires. The nanowire lasers were fabricated by coupling a top-down and bottom-up methodology and achieved lasing at wavelengths below the GaN bandedge. © OSA 2015.

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TYPE Conference Poster YEAR 2015
ScopusOSTI

Gallium nitride nanotube lasers

Applied Physics Letters

Li, Changyi L.; Liu, Sheng L.; Hurtado, Antonio H.; Wright, Jeremy B.; Xu, Huiwen X.; Luk, Ting S.; Figiel, J.J.; Brener, Igal B.; Brueck, S.R.; Wang, George T.

Lasing is demonstrated from gallium nitride nanotubes fabricated using a two-step top-down technique. By optically pumping, we observed characteristics of lasing: a clear threshold, a narrow spectral, and guided emission from the nanotubes. In addition, annular lasing emission from the GaN nanotube is also observed, indicating that cross-sectional shape control can be employed to manipulate the properties of nanolasers. The nanotube lasers could be of interest for optical nanofluidic applications or application benefitting from a hollow beam shape.

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TYPE Journal Article YEAR 2015
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Results 1–50 of 84
Results 1–50 of 84