Search

The recent progress in III-nitride tunnel junction (TJ) contacts for optoelectronic devices is summarized in this review paper. Due to the rapid developments of various III-nitride based optoelectronic devices for different emerging applications, TJs are of interest for these devices to achieve more efficient device performance. This review paper first briefly describes the working principles of TJs and the advantages of employing TJs in optoelectronic devices. The significant advancements in III-nitride TJs realized by metalorganic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) are addressed, as well as their TJ device performances. Lastly, the critical challenges for employing III-nitride TJs in wide bandgap ultraviolet (UV) light emitters are discussed for future investigations. [ABSTRACT FROM AUTHOR]

The surface roughness on different length scales of In2O3(001) films is detrimental to potential semiconductor applications of this material. One type of surface roughness consists of square shaped islands with edge lengths on the micron scale. This islanding was observed in In2O3(001) films grown on Y-stabilized ZrO2(001), YSZ(001), by plasma-assisted molecular beam epitaxy under oxygen-rich and under In-rich growth conditions. Our reflection high-energy electron diffraction observations during the initial phase of the growth showed that the islanding is caused by a nucleation of the In2O3 with incomplete wetting of the substrate and not by a strain-induced breakup of the film. By systematic variation in the growth parameters Indium-to-oxygen beam flux ratio (VI/III ratio) and substrate temperature we found that the wetting of YSZ(001) by the In2O3(001) film is thermodynamically not preferred and that kinetic limitations, such as low substrate temperatures or (to a lesser extent) high VI/III ratios, were able to force nucleation of continuous films. A growth diagram was constructed that shows the parameter regions with forced or inhibited wetting. The nucleation with inhibited wetting can be applied to the formation of square shaped In2O3(001) islands whose size increases and areal density decreases with decreasing VI/III ratio. To realize continuous, smooth In2O3(001) films under In-rich conditions, a thin nucleation layer was grown at lower substrate temperature to completely wet the substrate. The bulk of the film was grown on top of this nucleation layer at higher temperature to ensure a high crystalline quality. [ABSTRACT FROM AUTHOR]

A comprehensive study of Sn doping in In2O3 during plasma-assisted molecular beam epitaxy (PA-MBE) is given, covering growth aspects and application-relevant aspects such as structural and transport properties. Single crystalline, (001) oriented indium oxide (In2O3) thin films were grown on Y-stabilized ZrO2(001) and systematically doped with 1018 cm−3 to 6 × 1021 cm−3 tin (Sn) by PA-MBE. The Sn incorporation was proportional to the Sn flux up to a Sn concentration of ≈1020 cm−3 indicating well-controllable doping in this regime. Toward higher Sn concentrations the Sn incorporation was increasingly impeded, which could be somewhat mitigated by increasing the oxygen-to-indium flux ratio. The surface faceting of undoped In2O3(001) during growth under oxygen rich conditions was prevented by doping to Sn concentrations [ABSTRACT FROM AUTHOR]

The InAlN alloy system has bandgap energies that span the entire visible range and is a promising candidate for bandgap engineering. To facilitate bandgap engineering, the bandgap bowing for the entire composition range must be well characterized. In rich InxAl1−xN (x > 0.60), films were grown via plasma assisted molecular beam epitaxy on freestanding GaN substrates. The InxAl1−xN film compositions were determined using high resolution X-ray diffraction. On-axis 0002 ω-2θ scans were used to determine the c spacing of the films, reciprocal space maps of the 1 ¯ 015 peaks were used to determine that the films were completely relaxed, and the film compositions were calculated from that information assuming Vegard's law. The bandgaps were independently measured using absorbance spectroscopy and fitting to the linear region of a Tauc plot (dependence of α2 hν on hν) as determined by interpolating the second derivative of the data and selecting a region of low curvature. Compared with the literature, the data were fit to a composition-independent bandgap bowing model, and a bowing parameter of b = 4.0 ± 0.2 eV was calculated, which is consistent with previous results. [ABSTRACT FROM AUTHOR]

Deep level defects were characterized in Ge-doped (010) β-Ga2O3 layers grown by plasma-assisted molecular beam epitaxy (PAMBE) using deep level optical spectroscopy (DLOS) and deep level transient (thermal) spectroscopy (DLTS) applied to Ni/β-Ga2O3:Ge (010) Schottky diodes that displayed Schottky barrier heights of 1.50 eV. DLOS revealed states at EC − 2.00 eV, EC − 3.25 eV, and EC − 4.37 eV with concentrations on the order of 1016 cm−3, and a lower concentration level at EC − 1.27 eV. In contrast to these states within the middle and lower parts of the bandgap probed by DLOS, DLTS measurements revealed much lower concentrations of states within the upper bandgap region at EC − 0.1 – 0.2 eV and EC − 0.98 eV. There was no evidence of the commonly observed trap state at ∼EC − 0.82 eV that has been reported to dominate the DLTS spectrum in substrate materials synthesized by melt-based growth methods such as edge defined film fed growth (EFG) and Czochralski methods [Zhang et al., Appl. Phys. Lett. 108, 052105 (2016) and Irmscher et al., J. Appl. Phys. 110, 063720 (2011)]. This strong sensitivity of defect incorporation on crystal growth method and conditions is unsurprising, which for PAMBE-grown β-Ga2O3:Ge manifests as a relatively “clean” upper part of the bandgap. However, the states at ∼EC − 0.98 eV, EC − 2.00 eV, and EC − 4.37 eV are reminiscent of similar findings from these earlier results on EFG-grown materials, suggesting that possible common sources might also be present irrespective of growth method. [ABSTRACT FROM AUTHOR]

Using x-ray photoemission spectroscopy (XPS) and current-voltage (I-V) measurements of Hg contacts we show that the surface electron accumulation layer of In2O3 can be removed by an oxygen plasma treatment. For the untreated sample, XPS measured a downward band bending toward the surface and a conduction band peak, and the I-V curve was linear which indicated the presence of a surface accumulation layer. After the treatment an upward bending, the absence of the conduction band peak, and a nonlinear I-V curve indicated the absence of the surface accumulation layer. The sheet resistance of the surface accumulation layer of >45 kΩ was deduced from the increase of the total sheet resistance upon the treatment. The removal of the surface electron accumulation layer opens up the possibility to use Schottky contacts for electrical characterization and device applications of semiconducting In2O3. [ABSTRACT FROM AUTHOR]

Laser assisted atom probe tomography is used to characterize the alloy distribution in BGaN. The effect of the evaporation conditions applied on the atom probe specimens on the mass spectrum and the quantification of the III site atoms is first evaluated. The evolution of the Ga++/Ga+ charge state ratio is used to monitor the strength of the applied field. Experiments revealed that applying high electric fields on the specimen results in the loss of gallium atoms, leading to the overestimation of boron concentration. Moreover, spatial analysis of the surface field revealed a significant loss of atoms at the center of the specimen where high fields are applied. A good agreement between X-ray diffraction and atom probe tomography concentration measurements is obtained when low fields are applied on the tip. A random distribution of boron in the BGaN layer grown by molecular beam epitaxy is obtained by performing accurate and site specific statistical distribution analysis. [ABSTRACT FROM AUTHOR]

In this work, we report on the growth of (001) β-(AlxGa1−x)2O3 films in molecular beam epitaxy via metal oxide-catalyzed epitaxy. Films with Al contents up to 15% were grown and the Al content was measured with atom probe tomography. A relationship between the Al content and the out-of-plane lattice parameter was derived for both (001) and (100) orientations. Transmission electron microscopy showed no evidence of extended defects in (001) β-(AlxGa1−x)2O3, and reciprocal space maps confirmed that β-(AlxGa1−x)2O3 films were coherently strained to (001) β-Ga2O3. Sn was also demonstrated to act as a surfactant for (001) β-(AlxGa1−x)2O3 growth, allowing for high-quality, uniform films with smooth morphologies. [ABSTRACT FROM AUTHOR]

In this work, we report the growth of Mg-doped (010) β-Ga2O3 via plasma-assisted molecular beam epitaxy. Mg concentrations from 2 × 1016 to 8 × 1020 cm−3 with sharp doping profiles were realized. The Mg doping incorporation in β-Ga2O3 showed little dependence on the growth temperature and Ga:O flux ratio. Annealing at temperatures from 925 to 1050 °C resulted in significant diffusion, thus limiting the application of Mg-doped β-Ga2O3 to lower temperature growth techniques and processing. Mg accumulation near the sample surface after diffusion gives insight into the rich point–defect interaction that may play an important role in Mg diffusion. [ABSTRACT FROM AUTHOR]

The polarity of GaN grown by plasma-assisted molecular beam epitaxy was inverted from N-face to Ga-face by inserting a composite AlN/aluminum oxide (AlOx) interlayer structure at the inversion interface. The change in polarity was verified in situ by reflection high energy electron diffraction via intensity transients and postgrowth surface reconstructions, and ex situ by convergent beam electron diffraction and etch studies in an aqueous potassium hydroxide solution. The inverted materials showed smooth surfaces and good electrical properties. AlGaN/GaN high electron mobility transistors fabricated on the inverted epilayers showed good dc and high frequency performance. A current-gain cutoff frequency (fT) of 21 GHz and maximum oscillation frequency (fmax) of 61 GHz were measured in devices with a gate length of 0.7 μm. These data compare favorably to those of Ga-face AlGaN/GaN devices with a similar structure grown on Si-face SiC substrates. [ABSTRACT FROM AUTHOR]

Mg-doped indium oxide (In2O3) thin films were grown by plasma-assisted molecular beam epitaxy with Mg-concentrations ranging from 1017 to 6×1020 cm-3. In this concentration range Mg was incorporated into In2O3 without discernable impediment nor formation of secondary phases. Despite the role of Mg as acceptor, the films were n-type conductive in the as-grown state or after annealing in vacuum. For Mg-concentrations well in excess of the unintentional donor concentration annealing in oxygen resulted in semi-insulating films without detectable p-type conductivity. These results strongly suggest oxygen vacancies to act as shallow donors in In2O3 that can overcompensate the Mg acceptors. [ABSTRACT FROM AUTHOR]

We report ultra-high responsivity of epitaxial (Sn x Ga 1 − x) 2 O 3 (TGO) Schottky UV-C photodetectors and experimentally identified the source of gain as deep-level defects, supported by first principles calculations. Epitaxial TGO films were grown by plasma-assisted molecular beam epitaxy on (−201) oriented n-type β- Ga 2 O 3 substrates. Fabricated vertical Schottky devices exhibited peak responsivities as high as 3.5 × 10 4 A/W at −5 V applied bias under 250 nm illumination with sharp cutoff shorter than 280 nm and fast rise/fall time in milliseconds order. Hyperspectral imaging cathodoluminescence (CL) spectra were examined to find the mid-bandgap defects, the source of this high gain. Irrespective of different tin mole fractions, the TGO epilayer exhibited extra CL peaks at the green band (∼2.20 eV) not seen in β- Ga 2 O 3 along with enhancement of the blue emission-band (∼2.64 eV) and suppression of the UV emission-band. Based on hybrid functional calculations of the optical emission expected for defects involving Sn in β- Ga 2 O 3 , VGa–Sn complexes are proposed as potential defect origins of the observed green and blue emission-bands. Such complexes behave as acceptors that can efficiently trap photogenerated holes and are predicted to be predominantly responsible for the ultra-high photoconductive gain in the Sn-alloyed Ga 2 O 3 devices by means of thermionic emission and electron tunneling. Regenerating the VGa–Sn defect complexes by optimizing the growth techniques, we have demonstrated a planar Schottky UV-C photodetector of the highest peak responsivity. [ABSTRACT FROM AUTHOR]

We demonstrate the growth of epitaxial (100)-oriented, rutile Sb-doped SnO 2 (ATO) and (111)-oriented, cubic Sn-doped In 2 O 3 (ITO) transparent conducting oxide (TCO) contacts on top of an InGaN/GaN(0001) light emitting diode (LED) by plasma-assisted molecular beam epitaxy (PAMBE). Both oxides form rotational domains. The in-plane epitaxial alignment of the two ITO(111) rotational domains to the GaN(0001) was: GaN [21-10]|| ITO Domain1 [‐ 211]|| ITO Domain2 [‐ 1‐12]. A growth temperature as low as 600 °C was necessary to realize a low contact resistance between ATO and the top p-GaN layer of the LED but resulted in non-optimal resistivity (3.4 × 10 − 3 Ω cm) of the ATO. The current–voltage characteristics of a processed LED, however, were comparable to that of a reference LED with a standard electron-beam evaporated ITO top contact. At short wavelengths, the optical absorption of ATO was lower than that of ITO, which is beneficial even for blue LEDs. Higher PAMBE growth temperatures resulted in lower resistive ATO but higher contact resistance to the GaN, likely by the formation of an insulating Ga 2 O 3 interface layer. The ITO contact grown by PAMBE at 600 °C showed extremely low resistivity (10 − 4 Ω cm) and high crystalline and morphological quality. These proof-of-principle results may lead to the development of epitaxial TCO contacts with low resistivity, well-defined interfaces to the p-GaN to help minimize contact losses, and enable further epitaxy on top of the TCO. [ABSTRACT FROM AUTHOR]

InAlN dipole diodes were developed and fabricated on both (0001) Ga-Face and (000...) N-face oriented GaN on sapphire templates by molecular beam epitaxy. The orientation and direction of the InAlN polarization dipole are functions of the substrate orientation and composition, respectively. Special consideration was taken to minimize growth differences and impurity uptake during growth on these orientations of opposite polarity. Comparison of devices on similarly grown structures with In compositions in excess of 50% reveals that dipole diodes shows poorer forward bias performance and exhibited an increase in reverse bias leakage, regardless of orientation. Similarly, (0001) Ga-face oriented InAlN at a lowered 40% In composition had poor device characteristics, namely, the absence of expected exponential turn on in forward bias. By contrast, at In compositions close to 40%, (000...) N-face oriented InAlN devices had excellent performance, with over five orders of magnitude of rectification and extracted barrier heights of 0.53- 0.62 eV; these values are in close agreement with simulation. Extracted ideality factors ranging from 1.08 to 1.38 on these devices are further evidence of their optimal performance. Further discussion focuses on the growth and orientation conditions that may lead to this discrepancy yet emphasizes that with proper design and growth strategy, the rectifying dipole diodes can be achieved with InAlN nitride dipole layers. [ABSTRACT FROM AUTHOR]

The design of isotype InGaN/GaN heterobarrier diode structures grown by ammonia molecular beam epitaxy is presented. On the (0001) Ga-polar plane, a structure consisting of a surface n+ GaN contact layer, followed by a thin InGaN layer, followed by a thick unintentionally doped (UID) GaN layer, and atop a buried n+ GaN contact layer induces a large conduction band barrier via a depleted UID GaN layer. Suppression of reverse and subthreshold current in such isotype barrier devices under applied bias depends on the quality of this composite layer polarization. Sample series were grown under fixed InGaN growth conditions that varied either the UID GaN NH3 flow rate or the UID GaN thickness, and under fixed UID GaN growth conditions that varied InGaN growth conditions. Decreases in subthreshold current and reverse bias current were measured for thicker UID GaN layers and increasing InGaN growth rates. Temperature-dependent analysis indicated that although extracted barrier heights were lower than those predicted by 1D Schrödinger Poisson simulations (0.9 eV-1.4 eV for In compositions from 10% to 15%), optimized growth conditions increased the extracted barrier height from ~11% to nearly 85% of the simulated values. Potential subthreshold mechanisms are discussed, along with those growth factors which might affect their prevalence. [ABSTRACT FROM AUTHOR]

Unipolar-light emitting diode like structures were grown by NH3 molecular beam epitaxy on c plane (0001) GaN on sapphire templates. Studies were performed to experimentally examine the effect of random alloy fluctuations on electron transport through quantum well active regions. These unipolar structures served as a test vehicle to test our 2D model of the effect of compositional fluctuations on polarization-induced barriers. Variables that were systematically studied included varying quantum well number from 0 to 5, well thickness of 1.5 nm, 3 nm, and 4.5 nm, and well compositions of In0.14Ga0.86N and In0.19Ga0.81N. Diode-like current voltage behavior was clearly observed due to the polarization-induced conduction band barrier in the quantum well region. Increasing quantum well width and number were shown to have a significant impact on increasing the turn-on voltage of each device. Temperature dependent IV measurements clearly revealed the dominant effect of thermionic behavior for temperatures from room temperature and above. Atom probe tomography was used to directly analyze parameters of the alloy fluctuations in the quantum wells including amplitude and length scale of compositional variation. A drift diffusion Schrödinger Poisson method accounting for two dimensional indium fluctuations (both in the growth direction and within the wells) was used to correctly model the turn-on voltages of the devices as compared to traditional 1D simulation models. [ABSTRACT FROM AUTHOR]

In this work, we demonstrate the detailed optimization of metalorganic chemical vapor deposition (MOCVD)-grown tunnel junctions (TJs) utilizing selective area growth (SAG) for regular size (0.1 mm2) and micro-size InGaN light-emitting diodes (LEDs and µLEDs). Finite-difference time-domain simulations show that the SAG apertures result in a more directional light emission of far-field radiation pattern for the SAG TJ LEDs grown on patterned sapphire substrate. Moreover, it is noted that the n-InGaN insertion layer and Si-doped concentration in the n+GaN TJs layer is essential to realize a low forward voltage (Vf) in TJs LEDs. For both 0.1 mm2 LEDs and µLEDs, the Vf is independent on the SAG aperture space varied from 3 to 8 µm when the Si-doping level in the n+GaN layer is as high as 1.7 × 1020 cm−3. The optimized TJ LEDs exhibit a comparable differential resistance of 1.0 × 10−2 Ω cm2 at 100 A cm−2 and a very small voltage penalty of 0.2–0.3 V compared to the conventional indium tin oxide contact LEDs. The low Vf penalty is caused by a higher turn on voltage, which is the smallest one among the MOCVD-grown TJs LEDs and comparable to the best molecular beam epitaxy-grown TJs LEDs. [ABSTRACT FROM AUTHOR]

Tunnel junctions (TJs) offer alternative designs and promise in some cases improved performances for nitride-based light-emitting diode (LEDs) and laser diodes (LDs) and are widely used in academic studies. However, the voltage penalty of the LEDs and LDs, in comparison with standard contact technologies, has been a major concern especially for commercial applications. In this study, we investigated methods to achieve low excess voltage. Using ammonia molecular beam epitaxy (NH3 MBE), GaN TJs were grown on commercial metalorganic chemical vapor deposition (MOCVD) grown blue LED wafers. Atom probe tomography (APT) and secondary ion mass spectrometry (SIMS) indicate 1 min buffered HF (BHF) clean of the regrowth interface reduced Mg and impurity incorporation into the n++ regrown TJ layers. The wafers were processed and measured in parallel to reference wafers using both university processes and industry processes. At 20 A cm−2, TJ LEDs grown with Si δ-doping at the junction interface processed in the university cleanroom had a forward voltage of 3.17 V in comparison to 2.86 V for LEDs processed with a standard indium tin oxide (ITO) contact. Unencapsulated TJ LEDs processed by industrial process without ITO or current blocking layer had about 0.3 V excess voltage compared to reference LEDs. The TJ LEDs also had more uniform light emission profile. The low excess voltage and consistent results acquired in both settings suggest that TJ can be scaled for industrial processes. [ABSTRACT FROM AUTHOR]

Sn doping of (010) β-Ga2O3 grown by conventional plasma-assisted molecular beam epitaxy (PAMBE) and via metal oxide catalyzed epitaxy (MOCATAXY) using a supplied indium flux during molecular beam epitaxy (MBE) growth was investigated. While high Sn concentrations were achievable over a range of growth conditions in conventional PAMBE, Sn doping less than 1019 cm−3 resulted in non-uniform doping profiles for constant Sn cell temperatures, as well as run-to-run variation in doping. Sn doping in MOCATAXY grown β-Ga2O3 allowed for sharp doping profiles and a wide range of donor concentrations from 3.9 × 1016 cm−3 to 2 × 1019 cm−3 and a maximum room temperature Hall mobility of 136 cm2/V s at 3.9 × 1016 cm−3. From temperature-dependent Hall measurements, Sn was found to have a relatively deep donor state at 77 meV below the conduction band edge. The samples showed low electron mobility at cryogenic temperatures, suggesting the existence of high background impurity levels in the MBE grown films and the need for impurity control in the oxide MBE growth environment. [ABSTRACT FROM AUTHOR]

The dependence of electron mobility on growth conditions and threading dislocation density (TDD) was studied for n--GaN layers grown by ammonia-based molecular beam epitaxy. Electron mobility was found to strongly depend on TDD, growth temperature, and Si-doping concentration. Temperature-dependent Hall data were fit to established transport and charge-balance equations. Dislocation scattering was analyzed over a wide range of TDDs (∼2×106 cm-2 to ∼2×1010 cm-2) on GaN films grown under similar conditions. A correlation between TDD and fitted acceptor states was observed, corresponding to an acceptor state for almost every c lattice translation along each threading dislocation. Optimized GaN growth on free-standing GaN templates with a low TDD (∼2×106 cm-2) resulted in electron mobilities of 1265 cm2/Vs at 296K and 3327 cm2/Vs at 113 K. [ABSTRACT FROM AUTHOR]

• Plasma-assisted molecular beam epitaxy growth of InGaN at highest N flux reported. • Highest reported growth rate of InGaN at 1.3 μm/h. • Higher growth rate enables more In incorporation at higher temperatures. • Provides path forward for efficient nitride LEDs by PAMBE. Growth of efficient III-N based light emitting devices by plasma assisted molecular beam epitaxy has been elusive, even though the technique has attractive advantages in comparison to metal organic chemical vapor deposition. Modern high-flux radio frequency plasma systems could remedy this issue by enabling growth of In x Ga 1-x N at higher temperatures than previously possible, likely improving the material quality. In this work, active nitrogen fluxes of up to 3.5 μm/h GaN-equivalent growth rate were employed to grow In x Ga 1-x N alloys. In x Ga 1-x N growth rates of 1.3 μm/h were demonstrated at growth temperatures of 550 °C and 600 °C with maximum film compositions of In 0.25 Ga 0.75 N and In 0.21 Ga 0.79 N, respectively. A composition of In 0.05 Ga 0.95 N was observed in a film grown at 700 °C with smooth step-terrace morphology. [ABSTRACT FROM AUTHOR]

Epitaxial growth of β-Ga2O3 films on (110) substrates has been performed via plasma-assisted molecular beam epitaxy (PAMBE). The atomic force microscopy scan shows a very low root mean square roughness of 0.08 nm for the surface of the as-received (110) substrates. High-resolution x-ray diffraction measurements reveal a 2.5 nm/min growth rate of β-Ga2O3 films on (110) substrates for conventional PAMBE growth conditions (∼700 °C), which is comparable to that on (010) substrates. The surface morphology of β-Ga2O3 epitaxial films is smooth and has a similar dependence on Ga flux to (010) growth. However, the (110) plane does not have a tendency to show a well-defined step–terrace structure in spite of the appearance of (110) facets in the growth of (010) β-Ga2O3. Indium catalyzed growth was also demonstrated to improve the growth rate up to 4.5 nm/min and increase the maximum growth temperature up to 900 °C of (110) β-Ga2O3. [ABSTRACT FROM AUTHOR]

We investigated the effect of AlN nucleation layers (NLs) on the structural and electrical properties of N-face GaN grown on C-face 6H-SiC substrates by plasma-assisted molecular beam epitaxy. The GaN films were characterized by secondary ion mass spectroscopy, x-ray diffraction, and transistor electrical measurements. It was found that an AlN NL grown in the N-rich regime was essential for realizing highly resistive GaN buffers. The mosaic structure of the GaN epilayers was systematically correlated with the AlN nucleation conditions. N-face high electron mobility transistors fabricated on these low-leakage buffers demonstrated the highest output power density at 4 GHz to date of 8.1 W/mm with an associated power-added efficiency of 54%. [ABSTRACT FROM AUTHOR]

Mid-infrared intersubband transitions are investigated in nonpolar m-plane and a-plane GaN/AlGaN multi-quantum well heterostructures. Nominally identical heterostructures were grown by ammonia molecular-beam epitaxy on free-standing m-plane and a-plane GaN substrates. A total of 12 well- and barrier-doped samples with intersubband transition energies in the range of 220–320 meV (wavelength range 3.8–5.6 μm) were grown. The intersubband absorption lines of the m-plane samples were 10–40% narrower than those of the a-plane samples, and a very narrow intersubband absorption linewidth of 38 meV (full width at half maximum) at a transition energy of approximately 250 meV (5 μm wavelength) was observed in an m-plane sample. Narrower intersubband absorption linewidths of m-plane samples can be explained by more abrupt heterostructure interfaces revealed by structural characterization, which is attributed to a higher stability of the m-plane compared to the a-plane. No significant difference in the intersubband absorption linewidth was observed between the barrier- and well-doped samples. [ABSTRACT FROM AUTHOR]

We report on the direct measurement of hot electrons generated in the active region of blue light-emitting diodes grown by ammonia molecular beam epitaxy by electron emission spectroscopy. The external quantum efficiency of these devices is <1% and does not droop; thus, the efficiency losses from the intrinsic, interband, electron–electron–hole, or electron–hole–hole Auger should not be a significant source of hot carriers. The detection of hot electrons in this case suggests that an alternate hot electron generating process is occurring within these devices, likely a trap-assisted Auger recombination process. [ABSTRACT FROM AUTHOR]

The polarity of GaN grown by plasma-assisted molecular beam epitaxy was inverted from N-face to Ga-face by simultaneously exposing the surface to Mg and activated N fluxes during a growth interruption at a reduced substrate temperature. Growth studies suggested that a MgxNy compound was responsible for inverting the crystal. The change in polarity was verified in situ by reflection high energy electron diffraction via GaN surface reconstructions, and ex situ by convergent beam electron diffraction and KOH etch studies. The surface of the inverted material showed smooth step flow features. Ga-face high electron mobility transistors with good dc and small signal performance were fabricated on the inverted epilayers. A drain-source current of 0.84 A/mm was measured at a gate-source voltage of +1 V. Current-gain cutoff and maximum oscillation frequencies of 22 and 53 GHz, respectively, were measured in these devices. The device performance is similar to that of Ga-face transistors with comparable dimensions. [ABSTRACT FROM AUTHOR]

The strain dependence of the free-exciton resonance energies in AlN epilayers is presented and the values are analyzed using an appropriate Hamiltonian assuming equibiaxial stress for the wurtzite crystal structure in order to obtain valence band parameters. Based on the results, we study the strain dependence of the valence band ordering, optical transition probability, and free-exciton binding energy. As a result of these calculations, the following strain-free values are obtained for the energy gap, averaged dielectric constants, and ordinary and extraordinary dielectric constants: Eg=6.095 eV at T=11 K, ε=7.87, ε⊥=7.33, and ε|=8.45, respectively. A brief discussion of the valence band ordering in bulk AlxGa1-xN is also presented. [ABSTRACT FROM AUTHOR]

We describe the development of N-polar GaN-based high electron mobility transistors grown by N2 plasma-assisted molecular beam epitaxy on C-face SiC substrates. High mobility AlGaN/GaN modulation-doped two-dimensional electron gas channels were grown, and transistors with excellent dc and small-signal performance were fabricated on these wafers. Large-signal dispersion was observed, and the trap states responsible for this were identified, and layer designs to remove the dispersive effects of these traps were demonstrated. Finally, an AlGaN-cap layer was used to reduce gate leakage in these devices, and a low-dispersion high breakdown voltage device was achieved. This detailed study of dispersion and leakage in N-polar GaN-based transistors establishes a technological base for further development of field effect devices based on N-polar III-nitrides. [ABSTRACT FROM AUTHOR]

We have investigated the Ga-adlayer mediated growth of GaN quantum dots at 707 °C on AlN (0001) by simultaneous use of in situ reflection high-energy electron diffraction and line-of-sight quadrupole mass spectrometry during rf-plasma assisted molecular beam epitaxy (PA-MBE). We have found that the Ga-adsorbate coverage of 1.0 ML (monolayer) is critical in the mediation of a fundamental change in the GaN Stranski-Krastanov (SK) growth mode on AlN. When the Ga-adsorbate coverage was less than 1.0 ML, the SK transition occurred during GaN growth. For larger Ga-adsorbate coverages, the SK transition occurred after the desorption of the Ga coverage in excess of 1.0 ML. We performed variable GaN coverage growth experiments followed by desorption of the Ga adsorbate and subsequent GaN thermal decomposition to determine that the critical GaN coverage for the SK transition was less than 2.0 ML under Ga-droplet PA-MBE growth conditions. [ABSTRACT FROM AUTHOR]

We have investigated the adsorption and subsequent desorption of Ga on (0001) GaN using simultaneous line-of-sight quadrupole mass spectrometry (QMS) and reflection high-energy electron diffraction (RHEED). The in situ QMS and RHEED desorption transient measurements demonstrate the Ga flux dependent accumulation of the theoretically predicted laterally contracted Ga bilayer [J. E. Northrup et al., Phys. Rev. B 61, 9932 (2000)] under conditions similar to those used during GaN growth by rf-plasma molecular beam epitaxy. We correlated bioscillatory RHEED desorption transients [C. Adelmann et al., J. Appl. Phys. 91, 9638 (2002)] to QMS-measured Ga-adsorbate coverage and found both to be consistent with layer-by-layer desorption of the Ga-adsorbate bilayer. The QMS-measured steady-state Ga-adlayer coverage exhibited a continuous increase from 0 to 2.4 ML (monolayer) with respect to impinging Ga flux at substrate temperatures of 640–700 °C. We observed an exponential dependence of the Ga flux corresponding to 1.0 ML Ga-adsorbate coverage on substrate temperature and we measured an apparent activation energy of 2.43±0.11 eV and an attempt prefactor of 6.77×1012 nm/min (4.36×1011 Hz) for this transition. [ABSTRACT FROM AUTHOR]

Transition metals, such as Fe, are commonly used in either layers or substrates to serve as deep intentional acceptors to realize semi-insulating substrates, regrowth interfaces, or buffer layers. The unintentional incorporation of the compensating acceptor in subsequent layers is a major concern in epitaxial growth. In this paper, we report on unintentional Fe incorporation for the homoepitaxial growth of (010) β-Ga2O3 by plasma-assisted molecular beam epitaxy on (010) Fe-doped β-Ga2O3 substrates. Fe was found to incorporate heavily into films grown at 500 °C, while growth temperatures of 650 °C and higher showed a significantly longer tail of Fe in the films. This Fe tail was determined to be a result of surface riding during growth rather than diffusion. The total surface riding concentration of Fe was found to be approximately 3 × 1012 cm−2 from a typical Fe-doped (010) β-Ga2O3 substrate. Surface segregation coefficients of 0.982 and 0.993 were calculated for growth temperatures of 500 °C and 700 °C, respectively. Furthermore, growth temperatures of 500 °C–700 °C demonstrated high crystalline quality and smooth surface morphology. [ABSTRACT FROM AUTHOR]

Abstract Gallium Nitride (GaN) grown by ammonia molecular beam epitaxy and doped with elemental Germanium (Ge) is presented. Growth studies varying the GaN growth rate, substrate growth temperature and the elemental Ge flux reveal several incorporation dependencies. Ge incorporation increases with flux, as expected, and a doping range from ∼1017 cm−3 to 1020 cm−3 was readily achieved. A strong substrate temperature dependence on the electrical properties of films grown is observed, with an optimal growth temperature of 740 °C, lower than standard GaN growth conditions for ammonia molecular beam epitaxy. Compensation effects at higher growth temperatures are suspected, as observed with other techniques. Crystallographic defects are apparent at the highest doping concentrations from electrical and optical measurements, however thin layers of such highly doped films are of great interest for contact layers and tunnel junctions in devices. [ABSTRACT FROM AUTHOR]

We have studied the properties of Si, Ge shallow donors and Fe, Mg deep acceptors in β-Ga2O3 through temperature dependent van der Pauw and Hall effect measurements of samples grown by a variety of methods, including edge-defined film-fed, Czochralski, molecular beam epitaxy, and low pressure chemical vapor deposition. Through simultaneous, self-consistent fitting of the temperature dependent carrier density and mobility, we are able to accurately estimate the donor energy of Si and Ge to be 30 meV in β-Ga2O3. Additionally, we show that our measured Hall effect data are consistent with Si and Ge acting as typical shallow donors, rather than shallow DX centers. The high temperature Hall effect measurement of Fe doped β-Ga2O3 indicates that the material remains weakly n-type even with the Fe doping, with an acceptor energy of 860 meV relative to the conduction band for the Fe deep acceptor. Van der Pauw measurements of Mg doped Ga2O3 indicate an activation energy of 1.1 eV, as determined from the temperature dependent conductivity. [ABSTRACT FROM AUTHOR]

The homoepitaxial growth of N-polar GaN was investigated by ammonia molecular beam epitaxy. Systematic growth studies varying the V/III flux ratio and the growth temperature indicated that the strongest factor in realizing morphologically smooth films was the growth temperature; N-face films needed to be grown approximately 100 °C or greater than Ga-face films provided the same metal flux. Smooth N-face films could also be grown at temperatures only 50 °C greater than Ga-face films, albeit under reduced metal flux. Too high a growth temperature and too low a metal flux resulted in dislocation mediated pitting of the surface. The unintentional impurity incorporation of such films was also studied by secondary mass ion spectroscopy and most importantly revealed an oxygen content in the mid 10 17 to the mid 10 18  cm −3 range. Hall measurements confirmed that this oxygen impurity resulted in n-type films, with carrier concentrations and mobilities comparable to those of intentionally silicon doped GaN. [ABSTRACT FROM AUTHOR]

InAlN grown by plasma-assisted molecular beam epitaxy often contains a honeycomb microstructure. The honeycomb microstructure consists of 5–10 nm diameter aluminum-rich regions which are surrounded by indium-rich regions. Layers without this microstructure were previously developed for nominally lattice-matched InAlN and have been developed here for higher indium content InAlN. In this study, InAlN was grown in a nitrogen-rich environment with high indium to aluminum flux ratios at low growth temperatures. Samples were characterized by high-resolution x-ray diffraction, atomic force microscopy, high-angle annular dark-field scanning transmission electron microscopy, and atom probe tomography. Atomic force microscopy showed InAlN layers grown at temperatures below 450 °C under nitrogen-rich conditions were free of droplets. InAlN films with indium contents up to 81% were grown at temperatures between 410 and 440 °C. High-angle annular dark-field scanning transmission electron microscopy and atom probe tomography showed no evidence of honeycomb microstructure for samples with indium contents of 34% and 62%. These layers are homogeneous and follow a random alloy distribution. A growth diagram for InAlN of all indium contents is reported. [ABSTRACT FROM AUTHOR]

A PAMBE (plasma-assisted molecular beam epitaxy) growth diagram of InGaN is established for the metal-face (0001) orientation at 575 °C. This growth diagram allows identifying different growth regimes during the epitaxy of InGaN, namely the N-rich, the Ga-rich, the intermediate In-rich, and the In-accumulation regimes. InGaN compositional grades are then grown by ramping the Ga and In fluxes in parallel to remain in either the intermediate In-rich or the N-rich regime. The two samples exhibit distinct structural characteristics, surface morphologies, and optical properties. The growth diagram proves to be a useful tool in controlling the metal fluxes and thereby the stoichiometry conditions on the surface during the growth of compositionally graded layers. [ABSTRACT FROM AUTHOR]

In this paper we demonstrate ultraviolet (UV) light emitting diodes (LEDs) grown on metamorphic AlGaN buffers on freestanding GaN ( 20 2 ¯ 1 ) substrates by ammonia assisted molecular beam epitaxy (MBE). Misfit and related threading dislocations were confined to the stress relaxed, compositionally graded buffer layers, and single quantum well devices emitting at 355, 310 and 274 nm were grown on top of the graded buffers. The devices showed excellent structural and electrical ( I – V ) characteristics. [ABSTRACT FROM AUTHOR]

We have examined the effect of an indium surfactant on the growth of p-type GaN by ammonia-based molecular beam epitaxy. p-type GaN was grown at temperatures ranging from 700 to 780 °C with and without an indium surfactant. The Mg concentration in all films in this study was 4.5-6 × 1019 cm-3 as measured by secondary ion mass spectroscopy. All p-type GaN films grown with an indium surfactant had higher p-type conductivities and higher hole concentrations than similar films grown without an indium surfactant. The lowest p-type GaN room temperature resistivity was 0.59 Ω-cm, and the highest room temperature carrier concentration was 1.6 × 1018 cm-3. Fits of the temperature-dependent carrier concentration data showed a one to two order of magnitude lower unintentional compensating defect concentration in samples grown with the indium surfactant. Samples grown at higher temperature had a lower active acceptor concentration. Improvements in band-edge luminescence were seen by cathodoluminescence for samples grown with the indium surfactant, confirming the trends seen in the Hall data. [ABSTRACT FROM AUTHOR]

The results of vertical transport through nitride heterobarrier structures grown by ammonia molecular beam epitaxy are presented. Structures are designed with binary layers to avoid the effects of random alloy fluctuations in ternary nitride barriers. The unintentional incorporation of Ga in the AlN growth is investigated by atom probe tomography and is shown to be strongly dependent on both the NH3 flowrate and substrate temperature growth parameters. Once nominally pure AlN layer growth conditions are achieved, structures consisting of unintentionally doped (UID) GaN spacer layers adjacent to a nominally pure AlN are grown between two layers of nþ GaN, from which isotype diodes are fabricated. Varying the design parameters of AlN layer thickness, UID spacer layer thickness, and threading dislocation density show marked effects on the vertical transport characteristics of these structures. The lack of significant temperature dependence, coupled with Fowler-Nordheim and/or Milliken-Lauritsen analysis, point to a prevalently tunneling field emission mechanism through the AlN barrier. Once flatband conditions in the UID layer are achieved, electrons leave the barrier with significant energy. This transport mechanism is of great interest for applications in hot electron structures. [ABSTRACT FROM AUTHOR]

The results of vertical transport through nitride heterobarrier structures grown by ammonia molecular beam epitaxy are presented. Structures are designed with binary layers to avoid the effects of random alloy fluctuations in ternary nitride barriers. The unintentional incorporation of Ga in the AlN growth is investigated by atom probe tomography and is shown to be strongly dependent on both the NH3 flowrate and substrate temperature growth parameters. Once nominally pure AlN layer growth conditions are achieved, structures consisting of unintentionally doped (UID) GaN spacer layers adjacent to a nominally pure AlN are grown between two layers of n+ GaN, from which isotype diodes are fabricated. Varying the design parameters of AlN layer thickness, UID spacer layer thickness, and threading dislocation density show marked effects on the vertical transport characteristics of these structures. The lack of significant temperature dependence, coupled with Fowler-Nordheim and/or Milliken-Lauritsen analysis, point to a prevalently tunneling field emission mechanism through the AlN barrier. Once flatband conditions in the UID layer are achieved, electrons leave the barrier with significant energy. This transport mechanism is of great interest for applications in hot electron structures. [ABSTRACT FROM AUTHOR]

AlGaN/GaN high electron mobility transistors (HEMTs) irradiated with 1.8-MeV protons show more relative degradation in RF power/current gain, cutoff frequency f_T, and maximum oscillation frequency fmax than DC transconductance. These result from radiation-induced increases in fast bulk and surface trap densities, as well as increasing impedance mismatch at high frequencies with increasing proton fluence. NH3-rich MBE devices show less degradation in DC transconductance, but more degradation in RF gain than Ga-rich devices. [ABSTRACT FROM PUBLISHER]

Metal-polar In0.17Al0.83N barriers, lattice-matched to GaN, were grown under N-rich conditions by plasma-assisted molecular beam epitaxy. The compositional homogeneity of these barriers was confirmed by plan-view high-angle annular dark-field scanning transmission electron microscopy and atom probe tomography. Metal-polar In0.17Al0.83N/(GaN)/(AlN)/GaN structures were grown with a range of AlN and GaN interlayer (IL) thicknesses to determine the optimal structure for achieving a low two-dimensional electron gas (2DEG) sheet resistance. It was determined that the presence of a GaN IL was necessary to yield a 2DEG sheet density above 2 × 1013 cm−2. By including AlN and GaN ILs with thicknesses of 3 nm and 2 nm, respectively, a metal-polar In0.17Al0.83N/GaN/AlN/GaN structure regrown on a GaN-on-sapphire template yielded a room temperature (RT) 2DEG sheet resistance of 163 Ω/□. This structure had a threading dislocation density (TDD) of ∼5 × 108 cm−2. Through regrowth on a free-standing GaN template with low TDD (∼5 × 107 cm−2), an optimized metal-polar In0.17Al0.83N/GaN/AlN/GaN structure achieved a RT 2DEG sheet resistance of 145 Ω/□ and mobility of 1822 cm2 V−1 s−1. High-electron-mobility transistors with output current densities above 1 A mm−1 were also demonstrated on the low-TDD structure. [ABSTRACT FROM AUTHOR]

Plasma-assisted molecular beam epitaxial (PAMBE) growth of gallium nitride (GaN) was explored with a novel modification of a commercially available nitrogen plasma source. The modified nitrogen plasma source enabled a dramatic increase in the flux of active nitrogen and thus a significantly higher growth rate than has been previously reported. GaN films were grown using N2 gas flow rates between 1 and 8sccm while varying the plasma source's RF forward power from 200 to 600W. The highest growth rate, and therefore the highest active nitrogen flux achieved was ~2.65μm/h. For optimized growth conditions the surfaces displayed a clear step-terrace structure with an average RMS roughness (3µm×3µm) on the order of 1nm. Secondary ion mass spectroscopy (SIMS) impurity analysis demonstrates oxygen and hydrogen incorporation of 1×1016 and 5×1016 respectively, comparable to the metal organic chemical vapor deposition (MOCVD) grown template layer. Initial un-optimized electron mobility measurements of 1µm thick GaN layers have shown a peak mobility of ~705cm2/Vs for an electron concentration of ~3.5×1016 cm−3. A revised universal growth diagram is proposed allowing the rapid determination of the metal flux needed to grow in a specific growth regime for any and all active nitrogen fluxes available. [ABSTRACT FROM AUTHOR]

N-face GaN was grown on free-standing GaN (0001̄ ) substrates at a growth rate of 1.5 μm/h using plasma-assisted molecular beam epitaxy. Difference in growth rate between (00011̄) and (0001) oriented GaN depends on nitrogen plasma power, and the (00011̄) oriented GaN had only 70% of the growth rate of the (0001) oriented GaN at 300W. Unintentional impurity concentrations of silicon, carbon, and oxygen were 2×1015, 2×1016, and 7×1016 cm–3, respectively. A growth diagram was constructed that shows the dependence of the growth modes on the difference in the Ga and active nitrogen flux, ΦGa–ΦN*, and the growth temperature. At high ΦGa–UN* (ΦGa⪢ΦN*), two-dimensional (step-flow and layer-by-layer) growth modes were realized. High growth temperature (780 °C) expanded the growth window of the two-dimensional growth modes, achieving a surface with rms roughness of 0.48 nm without Ga droplets. [ABSTRACT FROM AUTHOR]

Generalized infrared spectroscopic ellipsometry (GIRSE) was applied to Sb-doped rutile SnO2(101) films grown by plasma-assisted molecular beam epitaxy (PAMBE). Coupled longitudinal-optical (LO) phonon-plasmon (LPP) modes for the two principal polarization directions of the optically anisotropic SnO2 were identified and their frequencies were determined as a function of electron concentrations obtained by Hall effect measurements. The analysis of these modes yielded very accurate values for the plasma frequencies and finally the anisotropy of the electron effective masses as a function of carrier density. Comparison to Hall effect electron concentrations yielded a non-parabolicity of the conduction band. [ABSTRACT FROM AUTHOR]

Atom probe tomography was used to characterize AlN interlayers in AlGaN/AlN/GaN heterostructures grown by plasma-assisted molecular beam epitaxy (PAMBE), NH3-based molecular beam epitaxy (NH3-MBE), and metal-organic chemical vapor deposition (MOCVD). The PAMBE-grown AlN interlayer had the highest purity, with nearly 100% of group-III sites occupied by Al. The group-III site concentrations of Al for interlayers grown by NH3-MBE and MOCVD were ∼85% and ∼47%, respectively. Hall measurements were performed to determine the two-dimensional electron gas mobility and sheet concentration. Sheet concentrations were ∼25%-45% higher with molecular beam epitaxy than with MOCVD, and these results matched well with atom probe data. [ABSTRACT FROM AUTHOR]

AlxGa1-xN/GaN (x = 0.06, 0.12, 0.24) and AlGaN/AlN/GaN heterostructures were grown on 6 H-SiC, GaN-on-sapphire, and free-standing GaN, resulting in heterostructures with threading dislocation densities of ∼2 × 1010, ∼5 × 108, and ∼5 × 107 cm-2, respectively. All growths were performed under Ga-rich conditions by plasma-assisted molecular beam epitaxy. Dominant scattering mechanisms with variations in threading dislocation density and sheet concentration were indicated through temperature-dependent Hall measurements. The inclusion of an AlN interlayer was also considered. Dislocation scattering contributed to reduced mobility in these heterostructures, especially when sheet concentration was low or when an AlN interlayer was present. [ABSTRACT FROM AUTHOR]

AlGaN/AlN/GaN heterostructures were grown on 6H-SiC, GaN-on-sapphire, and free-standing GaN, resulting in heterostructures with threading dislocation densities of ∼2 × 1010, ∼5 × 108, and ∼5 × 107 cm-2, respectively. Growths were carried out under Ga-rich conditions by plasma-assisted molecular beam epitaxy to determine the influence of threading dislocation density on the sheet resistance of AlGaN/AlN/GaN heterostructures. High threading dislocation density was observed to significantly degrade Hall mobility. An AlGaN/AlN/GaN heterostructure with a ∼2 nm AlN interlayer and a threading dislocation density of ∼5 × 107 cm-2 achieved the very low room temperature sheet resistance of 175 Ω/[larger_open_square]. [ABSTRACT FROM AUTHOR]

Nominally lattice matched InAlN/GaN was grown by plasma-assisted molecular beam epitaxy, and the intrinsic microstructure was investigated via x-ray diffraction, transmission electron microscopy, and atom probe tomography. The InAlN showed a cellular structure, which was comprised of ∼10 nm wide Al-rich cores and ∼1 nm In-rich InAlN intercellular boundaries. Despite the strong laterally non-uniform In distribution, both vertical and lateral lattices are unperturbed by the cellular structure, as evidenced by strong thickness fringes in on-axis ω-2θ high resolution x-ray diffraction scans, coherence lengths derived from on-axis (0002) and off-axis ([formula]) ω-2θ high resolution x-ray diffraction scans, and a modified Williamson-Hall analysis for on-axis reflections. [ABSTRACT FROM AUTHOR]

InAlN lattice-matched to GaN was grown by molecular beam epitaxy (MBE) using ammonia as the nitrogen source. The alloy composition, growth conditions, and strain coherence of the InAlN were verified by high resolution x-ray diffraction ω-2θ scans and reciprocal space maps. Scanning transmission electron microscopy and energy-dispersive x-ray spectroscopy of the InAlN revealed the absence of lateral composition modulation that was observed in the films grown by plasma-assisted MBE. InAlN/AlN/GaN high electron mobility transistors with smooth surfaces were fabricated with electron mobilities exceeding 1600 cm2/Vs and sheet resistances below 244 Ω/sq. [ABSTRACT FROM AUTHOR]