NAMBE2016 Session MBE-TuA: Highly Mismatched Alloys (1:30 pm-3:00 pm)/Nitride Materials and Devices (3:30 pm-5:00 pm)

The II₃V₂ materials are a relatively unexplored family of semiconductors, several of which are composed of earth-abundant, environmentally-benign elements, examples being Zn₃N₂, Mg₃N₂ and Zn₃P₃. These materials have relatively complex multi-atom unit cells however their structure can be described rather simply as similar to the fluorite structure with one of the three interpenetrating fcc lattices being half occupied. We have grown Zn₃N₂ and Mg₃N₂ films on A-plane sapphire and (100) MgO substrates by plasma MBE, using conventional K-cells for the metals and an RF plasma source for nitrogen. The growth rate was monitored optically. Epitaxial single crystal films of Zn₃N₂ have been obtained at growth temperatures in the 140-180°C range. The bandgap was found to be 1.25-1.28 eV from optical absorption measurements and an electron Hall mobility as high as 395 cm²/Vs was observed. No photoluminescence was detected. The Mg₃N₂ films are yellow with a bandgap of 2.6 eV. In the case of Zn₃N₂ the optical properties of the thin films have been compared with the optical properties of commercial powders. The powders show similar absorption spectra to the MBE films and in addition show photoluminescence at room temperature. Zn₃N₂ oxidizes over time in air while unprotected Mg₃N₂ oxidizes almost immediately on exposure to air.

Modern semiconductor devices often contain thousands of epitaxial layers with rigorous demands on layer composition, thickness and growth rate stability in order to achieve good structural quality and the desired device performance. Previously we demonstrated a novel technique of Sb flux monitoring in a custom VG V90 MBE system. This method has been used successfully for the growth of many Sb-containing structures, including laser diodes, in the years since its introduction. In the present work, we report real-time in-situ group-III and As flux monitoring using ion gauges mounted in the same custom MBE system. This enables simultaneous monitoring of In, As and Sb during the growth of InAs/AlSb structures which are critical parts of both GaSb-based interband cascade lasers (ICLs) and InAs-based quantum cascade lasers (QCLs).

In-situ monitoring for a given cell is achieved by measuring the flux which bypasses the substrate when the shutter is open, using a custom monitoring port deliberately positioned to provide a view of the bypassing beam. Sb monitoring was achieved using a quadropole mass spectrometer (QMS), while As and In fluxes were separately monitored using standard Bayard-Alpert ionization gauges. The As flux signal has a long transient response. In contrast, we show exceptional response, sensitivity, stability and signal:noise for real-time In flux monitoring, even in the presence of large As background pressures during InAs growth. Future MBE system designs should incorporate similar features to take advantage of this possibility.

The molecular beam epitaxy growth and the structural and optical properties of bulk InAsSbBi and InAsSb/InAsSbBi/InAsSb quantum wells are examined using reflection high-energy electron diffraction, X‑ray diffraction, Rutherford backscattering spectrometry, spectroscopic ellipsometry, and photoluminescence spectroscopy. Figure 1a shows the Rutherford backscattering spectrum from a 210 nm thick layer of InAsSbBi grown on GaSb between two 10 nm thick AlSb barrier layers. There is an energy range (above 1.8 MeV) over which the backscattered ion energies are uniquely characteristic of Bi; as a result the Bi mole fraction of the InAsSbBi quaternary is determined to a high degree of accuracy. Furthermore, the peak in the backscattered Bi yield at 1.86 MeV indicates that surface Bi not incorporated into the InAsSbBi layer is incorporated into the upper AlSb barrier. Figure 1b shows the 004-plane X-ray diffraction pattern from the same sample. The sharpness of the InAsSbBi layer diffraction peak and the wide angular range over which Pendellösung fringes are observed indicates that the compressively strained InAsSbBi exhibits excellent structural quality. The angular separation between the substrate and layer diffraction peaks provides the tetragonal distortion from which the As and Sb mole fractions are uniquely determined, as the Bi mole fraction is independently determined using Rutherford backscattering. The bandgap energies measured by photoluminescence and spectroscopic ellipsometry are compared to those predicted by the valence band anticrossing model. These and additional results will be presented at the conference.

We demonstrate InAsSbN MQW photodiode and a p-i-n In(AsN)/(InAl)As resonant tunnelling diode (RTD) grown on InAs substrate by molecular beam epitaxy (MBE). Room temperature photoresponse is observed from InAsSbN/InAs multi-quantum well photodiodes in the mid-infrared spectral region. The extended long wavelength photoresponse is identified to originate from the InAsSbN e1-hh1 and e1-lh1 transitions, with a cut off wavelength ~ 4.20 µm and peak detectivity D* =1.25×10⁹ cm Hz^1/2 W^-1. We observe new type of Zener tunnelling in In(AsN)/(InAl)As resonant tunnelling diode (RTD) that involves the resonant transmission of electrons through zero-dimensional (0D) states. The incorporation of nitrogen in the quantum well creates 0D states that are localized on nanometer lengthscales. These levels provide intermediate states that act as “stepping stones” for electrons tunnelling across the diode and give rise to a negative differential resistance (NDR) that is weakly dependent on temperature. These electron transport properties have potential for the development of novel nanometre-scale non-linear components for electronics and MIR photonics.

In this paper, we propose GaAs_0.96Bi_0.04/GaAs (12 nm/12 nm) MQWs is good for optical devices such as LEDs and laser diodes (LDs), and demonstrates that LED shows better performance for long wavelength light emission with less Bi composition. Eleven periods of GaAs_0.96Bi_0.04/GaAs (12 nm /12 nm) MQWs were grown on n-type GaAs (100) substrate under an optimized growth condition. The substrate temperature was 350°C for GaAsBi and 550°C for GaAs layer. The total thickness of the MQW is 260 nm and enable us optical confinement of 1.2 mm long wavelength light using GaAs cladding layers. The MQWs i-region were stacked between 600 nm thick n-type and p–type GaAs layer.

The experimental and simulated XRD curves for GaAsBi MQWs fit returns with 4% Bi concentration. Photoluminescence (PL) at 10 K, sharp and single peak obtained from MQWs at 1.17 μm, with 64 meV narrow line width and it has small red shift with increasing temperature up to 1.23 μ m at RT. EL spectra for a GaAsBi/GaAs MQWs LED for various injection current densities at RT are shown in fig.1. Electroluminescence (EL) from the LED showed sharp and single peak at 1.23 μm, whose value is the same as the GaAaBi/GaAs (50 nm/3 nm) MQW LED with 6%-Bi-containing GaAsBi layers by Richards et al. [1]. The result strongly support that GaAsBi/GaAs has type-II band configuration as is reported in our previous paper[2]. FWHM of the EL peak is 100 meV. The value is 34 meV smaller than the GaAaBi/GaAs (50 nm /3 nm) MQW LED². Current and voltage curve is shown in fig.2, the forward voltage at an injection current of 130 mA is 3.3 V and current density is 983 A/cm², which comparably higher than the previously reported results[1]. This result indicates that GaAsBi/GaAs (12 nm / 12 nm) MQWs is advantageous for LEDs and LDs achieving longer wavelength emission with less Bi composition.

The industrial scalability of inorganic photonics via molecular beam epitaxy (MBE) is limited in large part limited by the expensive and rigid single crystalline substrates needed for epitaxial growth. Nanowires have a distinct advantage over their thin film counterparts; they can relieve strain extremely efficiently without the creation extended defects, opening a host of new substrate choices. Despite this, both planar and nanowire based solid-state photonic devices are still typically grown on single crystalline substrates. In this work, plasma-assisted MBE was used to create self-assembled AlGaN nanowires directly on flexible metal foils. No surface preparation steps were done on the Ti and Ta foils apart from standard solvent cleaning. Scanning electron microscopy measurements, coupled with electron backscatter diffraction, show that the nanowires are uniform and oriented parallel to each other inside the individual grains of the foils. Photoluminescence (PL) measurements of GaN nanowires grown on the foils have no long wavelength defect peaks and intensities comparable to the same structures on single crystal Si wafers. Likewise, the decay lifetimes, determined from time-resolved PL, do not vary significantly between the different substrates. Operational polarization-engineered nanowire LEDs are fabricated directly on Ta foils [1]. Current-voltage analysis reveals a threshold voltage of ~5 V, which is lower than similar devices on silicon. Electroluminescence spectra show peak emission at ~350 nm. These results open the door for large scale, roll-to-roll manufacturing of nanowire optoelectronics.

^ǂ Author for correspondence: myers.1078@osu.edu

[1] B. J. May, ATM G. Sarwar, and R. C. Myers, Appl. Phys. Lett. 108, 141103 (2016).

This work was supported by the Army Research Office (W911NF-13–1–0329) and the National Science Foundation CAREER award ( DMR-1055164).

With a large bandgap of 6.1 eV, AlN is a critical material for deep ultraviolet (UV) emitting devices. However, due to the lack of suitable lattice-matched substrate, the growth of high quality AlN has been extremely difficult. Furthermore, the very large activation energy of Mg and the resulting poor conductivity and low hole concentration of p-type AlN have severely limited the development of deep UV optoelectronic devices [1]. We have recently demonstrated that significantly enhanced Mg-dopant incorporation and efficient p-type conduction can be achieved in low dimensional nanostructures, such as nanowires [2]. We have further performed a detailed investigation of the molecular beam epitaxy, and structural, electrical and optical characterization of Mg-doped AlN nanowall structures to elucidate the mechanism of Mg-dopant incorporation in wide bandgap nanostructures and to demonstrate large area AlN LEDs.

In this study, Si-doped GaN template on sapphire substrate was firstly patterned with top-down approach, including electron beam lithography and dry etching, to fabricate nanowall arrays. 120 nm thick Mg-doped AlN layer was then grown on GaN to form an AlN nanowall structure by using a Veeco Gen II plasma-assisted molecular beam epitaxial system. The widths of AlN nanowalls were varied from 100 nm to 5 mm. A relatively low nitrogen flow was used to maintain a nearly metal-rich condition. Photoluminescence properties of Mg-doped AlN nanowall structures were measured with a 193 nm ArF excimer laser at room temperature. Strong emission peaks at 210 nm and 235 nm were measured, which correspond to free exciton emission in AlN and Mg-acceptor related transition, respectively. The direct measurement of Mg-acceptor related optical transition in AlN at room-temperature is unprecedented and suggests excellent material quality. Moreover, nanowall structures with narrower widths exhibit significantly enhanced optical emission, due to the reduced defect formation. Our detailed studies further suggest that Mg-dopant incorporation is significantly enhanced in AlGaN nanowall structures, which is explained by the reduced formation energy for Mg-dopant, due to the smaller strain distribution. The demonstration of AlN nanowall deep UV LEDs and lasers is currently in progress and will be reported.

In this work, we investigate the impact of polarization dopind (pi-doping) in n-type Al-rich AlGaN alloys to be used in sub-300 nm deep UV light emitting diodes (LEDs). To do this, polarization charge assisted doping schemes in the forms of linearly graded AlGaN alloys and short period GaN/AlN superlattices were grown by plasma assisted MBE on AlN templates. A control simple with conventional Si-doped n-type Al_0.5Ga_0.5N simple was grown as shown in Fig 1. The chemical composition of all samples were confirmed using HR-XRD measurements. The absorption bandedge was detected from UV-Vis measurement and all samples showed transparency to >280 nm (Fig. 2(a)). For electrical characterization, Ti/Al/Ni/Au metal contacts were put in a Van-der-pauw configuration and Hall measurement were carried out. Fig 2(b) shows the summary of such measurement. All samples show large sheet charge density, decent mobility and low resistance both at 300K and 77K. Further temperature dependent transport analysis, gated C-V and I-V measurements will be presented to understand such large charge densities in the samples.

We report a 250 nm deep UV light emitting diode (DUV LED) grown on high quality bulk AlN substrates from Hexatech with a threading dislocation density of 1x10⁴ cm^-2 by plasma assisted molecular beam epitaxy (PAMBE). The AlN substrates were sonicated in organic solvents outside the MBE, and baked at 200o C for 7 hrs followed by 450o C for 1.5 hrs in MBE vacuum before epitaxial growth at a thermocouple temperature of 750o C. A Nitrogen RF plasma power of 200 W corresponding to a growth rate of 0.17 ML/s was used. A 20 nm AlN nucleation layer was deposited on the bulk AlN substrate using migration enhanced epitaxy (MEE) technique which ensures a metal free smooth growth surface. A 110 nm linearly graded down buffer layer was grown after the AlN nucleation layer. For the n-contact region, a Si-doped 110 nm 77% Al_xGa_1-xN layer was grown. The composition of the AlGaN layer were determined from X-ray diffraction measurement from separate calibration growths. 3 periods of GaN/AlN heterostructure were inserted as the DUV light emitting region. For this active region, 2 ML thick GaN quantum-disks were grown in the Stranski-Krastanov mode between 2.5 nm AlN barriers. The disks were grown under Nitrogen-rich conditions. For the AlN barrier regions, MEE method was employed to ensure a smooth hetero-interface between GaN and AlN. For the p-contact region, polarization-induced doping technique was used to enhance the free hole concentration. A 60 nm thick linearly graded down Mg-doped AlGaN layer in which the Al% was graded down from 100%-77% was grown on the active region. To measure the impact of dislocations, the LED structures were simultaneously grown on co-loaded bulk AlN and AlN-on-Sapphire template. TEM analysis indicated that the desired thicknesses and compositions of all layers were achieved. Cl₂ based RIE etching was used to define MESAs for deposition of the p-contact (5 nm Ni/5 nm Au current spreading followed by 20 nm Ti/100 nm Au) and the n-contact (Ti/Al/Ni/Au-20/100/40/50 nm). A 250 nm DUV electroluminescence with a narrow 13 nm FWHM has been demonstrated. The I-V characteristics of DUV LED on the bulk AlN substrate exhibited far lower reverse leakage and sharp turn-on voltage than the I-V of the LED on the AlN-on-Sapphire template. A low turn-on voltage of ~4V was observed for the 250 nm LED on the bulk AlN substrate. A commercial software package SiLENSe was used to simulate the energy band diagram, J-V characteristics and the EL spectra. The simulated EL spectra agreed nicely with the experimental data. Though the simulated J-V curve deviated from the measured characteristics because of the large resistance of the access n-type AlGaN region.