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The reduction of undesirable gas phase reaction between trimethylaluminum and NH3 was achieved by spatially separating the precursors using N2 purge line during AlN growth by metal organic chemical vapor deposition (MOCVD). Under this condition, it was shown that the growth pressure has a strong impact on the surface morphology independent of pre-reaction. For 0.8-μm-thick AlN grown on (0001) sapphire substrates, increasing pressure from 200 to 500 Torr drastically increased a root-mean-squared surface (r.m.s.) roughness from 0.48 to 33 nm. This morphological change was previously attributed to the pre-reaction. Less pre-reaction also allowed us to investigate the pressure dependence of impurity (carbon and oxygen) incorporation in AlN as the growth rate was no longer affected by the pressure. Unlike GaN, the carbon level almost doubled with increasing pressure from 200 to 500 Torr. By optimizing the surface morphology (r.m.s. roughness from 33 to 0.62 nm) at 500 Torr, the carbon concentration in AlN decreased from 5 × 1018 to 7 × 1017 cm−3. Although there was no improvement in the structural quality, this uniquely designed MOCVD could further improve the material quality of AlN by reducing the impurity level.

We observe terahertz (THz) emission from c -plane InN and In0.15Ga0.85N films on GaN templates due to surface-normal transport with a strong anomalous in-plane transport component. Analysis of the rotational dependence of the THz emission associated with this in-plane transport indicates that an electric field exists along the [1-100] m-axis correlated with the underlying template miscut. Calculations show that the field is correlated with strain-induced polarization fields at the heterointerface related to step-like charges at interfaces of the epilayers and the miscut templates. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Heterogeneous aluminium gallium nitride (Alx Ga1-xN)/aluminium nitride (AlN)/silicon carbide (SiC) based n-i-p photodetectors have been demonstrated to effectively tailor the spectral response of SiC within the solar-blind regime. The differences in polarization at the hetero-interfaces resulting in negative polarization induced charge at the Alx Ga1-xN/AlN interface and positive polarization induced charge at the AlN/SiC interface has been exploited to create a large barrier to carrier transport across the interface. This barrier impedes the collection of photo-excited holes in the Alx Ga1-xN layers and enables the selective collection of electrons photo-excited to the Γ and L conduction band valleys of SiC while blocking the collection of electrons in the M valley. In this work, the influence of device design, including the AlN layer thickness and Alx Ga1-xN composition, on the spectral response is discussed. Thin AlN barrier layers are easily overcome by electrons generated in all valleys of 4H-SiC with increasing bias voltage while thicker barrier layers successfully minimize the collection of electrons in the M valley of SiC and therefore suppress the long-wavelength response >260 nm. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Mid-ultraviolet LEDs grown on AlN substrates with a range of quantum efficiencies and wavelengths spanning 250 to 280 nm have been investigated by time-resolved photoluminescence and electroluminescence. Through scaling of room temperature internal quantum efficiencies across all devices, radiative and nonradiative lifetimes are also estimated. General trends observed include an increase in PL lifetime for longer wavelength and higher external quantum efficiency devices, consistent with the increase in the estimated nonradiative life-time with increasing wavelength. Despite these trends, the external quantum efficiency of the devices increases only weakly with increasing wavelength from 258 to 279 nm, suggesting that optimization of radiative lifetime and injection efficiency also play an important role. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

This work demonstrates two novel 4H-SiC-based photodiode structures that enhance the response from ~200 nm to 260 nm by increasing the absorption of DUV photons within the high-electric-field depletion region and more efficiently collecting photo-generated carriers through drift as opposed to diffusion, despite the presence of surface recombination. In particular, the two devices discussed in this work have replaced the heavily doped, top-illuminated, n + -layer of conventional p-n - -n + diodes by a semi-transparent metal contact to create a p-n - -metal based device and by an n-type, wider bandgap AlGaN layer to create a heterojunction 4H-SiC/AlGaN p-n - -n + based device.

Temperature-dependent carrier transport is investigated using ultrafast spectroscopy in a p-GaN/i-InGaN/n-GaN solar cell with heavily-doped layers to compensate for polarization charges at the hetero-interface. We observe a flip in the transport direction at 110 K.

UVC light output of 66 mW at 300 mA CW has been achieved from LEDs on AlN substrates with extensive photon extraction. Proper vessel design allows for efficient irradiation of a water sample for purification.

Deep ultraviolet (DUV) avalanche photodiodes (APD) sensitive at wavelengths shorter than 260 nm have numerous applications, including chemical and biological identification and water quality monitoring. Wide bandgap materials such as silicon carbide (SiC) and the III-nitrides (AlInGaN) are well suited for UV detector applications. SiC devices have high avalanche gain, low dark current and low excess noise owing to a low k-factor (k=0.01) due to the much higher impact ionization coefficient for holes relative to electrons; however, these devices have poor response at wavelengths shorter than 260 nm related to carrier trapping by surface states. Conversely, the III-nitrides have a direct bandgap which can be engineered for efficient absorption in the DUV regime, but suffer from high leakage currents, especially for lattice mismatched growth, and difficulty in p-type doping in high AlN mole fraction alloys. To overcome these challenges, this work targets separate absorption and multiplication avalanche photodiodes (SAM-APDs) utilizing an aluminum gallium nitride (AlxGa1-xN) absorption layer and a SiC multiplication region.

Gallium Nitride (GaN) is a promising wide band gap semiconductor material for many optoelectronic applications, especially in the near UV range. Over the past several years, an extensive technical effort has been focused on improving the quality of GaN films through various overgrowth techniques such as epitaxial lateral overgrowth (ELOG), facet controlled epitaxial lateral overgrowth (FACELO), and Pendeoepitaxy. ELOG has been shown to reduce the density of threading dislocations by up to five orders of magnitude [1], however a complete physical model describing lateral overgrowth is needed in order to take full advantage of the process. A lateral overgrowth model will allow for the design and fabrication of three dimensional structures that can lead to novel devices and also to efficient biosensors by integrating micro and nano channels on the same chip as the optoelectronic components.A two-step process has been used to successfully control the geometry of overgrown GaN. Conditions have been identified which give a reduced lateral growth rate, in order to allow expansion of the {112n} plane to form vertical sidewalls and for the design of channel width. These geometries are being examined for possible application in laser diode and micro-channel fabrication for integrating bio-agent detection modules.

We present the analysis of the external quantum efficiency in AlGaN deep ultraviolet (DUV) light-emitting diodes (LEDs) on sapphire substrates and discuss factors affecting the output power of DUV LEDs. Performance of the LED is related to optimization of the device structure design and improvements of the epitaxial material quality.

High sensitivity deep ultraviolet (DUV) photodetectors sensitive at wavelengths shorter than 280 nm are useful for various applications, including chemical and biological identification, optical wireless communications, and UV sensing systems. State-of-the-art SiC APDs, employing a recessed top window, exhibit a high peak quantum efficiency (QE) of 60% at 268 nm, but the response of these devices diminishes at shorter wavelength due to increasing photo-generation of carriers in the top doped layer of this device, the short diffusion length of minority carriers in this doped region and the presence of a high density of surface states [1]. In this paper we explore two approaches for increasing the DUV photoresponse of 4H-SiC photodiodes by increasing the collection of DUV- generated carriers near the surface through drift within the high E-field region. As a result, the photo-generated carriers are more efficiently collected and the impact of surface state recombination is reduced. These approaches include replacing the illuminated top doped SiC layer of the diode with (1) a transparent metal contact or (2) a doped wider band gap III-Nitride semiconductor structure that is transparent in the spectral range of interest. Figure 1 shows the measured unity gain spectral response of a p-n--metal 4H-SiC diode reverse biased at 30V and consisting of a 2 µm-thick p +-SiC layer doped 2x1018 cm-3, a 350 nm-thick n- -SiC layer lightly doped 5x1015 cm-3, and employing a thin 5 nm transparent Ni metal top contact (broken red line). The response for a typical p-n--n+-SiC diode consisting of a 2 µm thick p +-SiC layer doped 2x1018 cm-3, a 480 nm thick n- -SiC layer lightly doped 5x1015 cm-3, and a 100 nm thick n +-SiC layer doped 4.5x1018 cm-3 is provided for comparison (solid-black line). A nearly flat QE between 40-45% is observed for the p-n--metal detector between 200- 270 nm that exceeds the performance of the conventional p-n--n+ diode for wavelengths shorter than 235 nm despite absorption losses within the metal contact across the spectral range of ~40-50%. This enhancement is attributed to improved collection of photogenerated carriers through drift within the high E-field region despite the presence of surface states. This conclusion is supported by modeling the expected photocurrent in the p-n- -metal (red open circles) and conventional p-n--n+ (black open squares) diodes. An alternative approach that can reduce absorption losses in the spectral range of interest is to employ a transparent III-Nitride semiconductor n-type doped layer. Figure 2 shows the measured spectral response of a photodiode having a 2 µm-thick p +-SiC layer doped 2x1018 cm-3, a 350 nm-thick n- -SiC layer lightly doped 5x1015 cm-3, a 10 nm-thick AlN layer, and a 100 nm-thick Al0.85Ga0.15N layer. The devices exhibit a peak unity gain QE of 55-58% between 265-250 nm that diminishes at shorter wavelength due to absorption in the top AlGaN layer. Increasing reverse bias above 40V results in an increase in response peaking around ~ 250 nm that is associated with inhomogeneous gain at wavelengths shorter than 255 nm (Figure 2- top). This is attributed to increased absorption associated with valence to higher conduction band transitions in SiC that ensures that these photons are absorbed near the interface between SiC and AlN, where photo-generated holes will have maximum transit distance through the depletion region as well as the larger impact ionization rate for holes over electrons for SiC. Calculations of the expected gain vs. wavelength within a p-i-n SiC diode illuminated from the n- region for varying magnitudes of E-field in the depletion region show similar trends consistent with this interpretation (Figure 2 inset). [1] Liu H., Mcintosh, D., Bai, X., Pan, H, Liu, M., Campbell J. C., and Cha H, IEEE Photonic Technol. Lett. 20, 1551 (2008).

Heterogeneous n-III-nitride/i-p silicon carbide (SiC) photodetectors have been demonstrated that enable the tailoring of the spectral response in the solar blind region below 280 nm. The negative polarization induced charge at the aluminum gallium nitride (AlxGa1−xN)/aluminum nitride (AlN) interface in conjunction with the positive polarization charge at the AlN/SiC interface creates a large barrier to carrier transport across the interface that results in the selective collection of electrons photoexcited to the Γ and L valleys of SiC while blocking the transport of electrons generated in the M valley. In addition, the AlxGa1−xN alloys act as transparent windows that enhance the collection of carriers generated by high energy photons in the fully depleted SiC absorption regions. These two factors combine to create a peak external quantum efficiency of 76% at 242 nm, along with a strong suppression of the long-wavelength response from 260 nm to 380 nm.

In this letter, the achievement of over 60 mW output power from pseudomorphic ultraviolet light-emitting diodes in continuous wave operation is reported. Die thinning and encapsulation improved the photon extraction efficiency to over 15%. Improved thermal management and a high characteristic temperature resulted in a low thermal rolloff up to 300 mA injection current with an output power of 67 mW, an external quantum efficiency (EQE) of 4.9%, and a wall plug efficiency (WPE) of 2.5% for a single-chip device emitting at 271 nm in continuous wave operation.

Enhanced terahertz emission from coherently strained InxGa1−xN epilayers on GaN is observed, which exceeds or is comparable to bulk InAs emission at pump wavelengths of 400 nm or 800 nm, respectively. The inverted terahertz waveform from the InxGa1−xN/GaN heterostructure indicates that the dominant terahertz generation mechanism is electron acceleration toward the InxGa1−xN surface in an internal electric field primarily associated with piezoelectric polarization charge at the heterointerface, rather than diffusive transport away from the surface typically observed in bulk semiconductors. The persistence of the inverted waveform for 266 nm excitation provides evidence of ultrafast electron relaxation via LO phonon emission.