Ammonia Molecular Beam Epitaxy Research Articles

Effects of growth temperature on Mg-doped GaN grown by ammonia molecular beam epitaxy

The hole concentration p in Mg-doped GaN films grown by ammonia molecular beam epitaxy depends strongly on the growth temperature TGR. At TGR=760 °C, GaN:Mg films showed a hole concentration of p=1.2 × 1018 cm−3 for [Mg]=4.5×1019 cm−3, while at TGR=840 °C, p=4.4×1016 cm−3 for [Mg]=7×1019 cm−3. Post-growth annealing did not increase p. The sample grown at 760 °C exhibited a low resistivity of 0.7 Ωcm. The mobility for all the samples was around 3−7 cm2/V s. Temperature-dependent Hall measurements and secondary ion mass spectroscopy suggest that the samples grown at TGR>760 °C are compensated by an intrinsic donor rather than hydrogen.

Growth regimes during homoepitaxial growth of GaN by ammonia molecular beam epitaxy

c-plane GaN films were grown by ammonia molecular beam epitaxy on metal-organic chemical vapor deposition templates for a wide range of NH3:Ga flux ratios and growth temperatures, and the resulting films were characterized using atomic force microscopy, reflection high-energy electron diffraction, and transmission electron microscopy. Three distinct nitrogen-rich growth regimes—unstable layer-by-layer, quasi-stable step flow, and dislocation-mediated pitting—were identified based on the growth mode and film properties. In addition, step flow growth was observed under conditions of gallium droplet accumulation. The results indicate the existence of two regimes for step-flow growth of GaN by ammonia MBE—both gallium-rich and nitrogen-rich. Growth mode instabilities and mound formation were observed and are discussed in the context of a step-edge energy barrier to adatom diffusion over a terrace.

Structural properties of GaN grown on AlGaN/AlN stress mitigating layers on 100-mm Si (111) by ammonia molecular beam epitaxy

The structural properties of GaN grown on AlGaN/AlN stress mitigating layers on 100-mm diameter Si (111) substrate by ammonia molecular beam epitaxy have been reported. High resolution X-ray diffraction, micro-Raman spectroscopy, transmission electron microscopy and secondary ion mass spectroscopy have been used to study the influence of AlN thickness and AlGaN growth temperature on the quality of GaN. GaN grown on thicker AlN showed reduced dislocation density and lesser tensile strain. Three-dimensional growth regime was observed for GaN grown at lower AlGaN growth temperature while higher AlGaN growth temperature resulted in two-dimensional growth mode. The dislocation bending and looping at the AlGaN/AlN interface was found to have significant influence on the dislocation density and strain in the GaN layer. The evolution and interaction of threading dislocations play a major role in determining the quality and the strain states of GaN.

Indium and impurity incorporation in InGaN films on polar, nonpolar, and semipolar GaN orientations grown by ammonia molecular beam epitaxy

The effects of NH3 flow, group III flux, and substrate growth temperature on indium incorporation and surface morphology have been investigated for bulk InGaN films grown by ammonia molecular beam epitaxy. The incorporation of unintentional impurity elements (H, C, O) in InGaN films was studied as a function of growth temperature for growth on polar (0001) GaN on sapphire templates, nonpolar (101¯0) bulk GaN, and semipolar (112¯2), (202¯1) bulk GaN substrates. Enhanced indium incorporation was observed on both (101¯0) and (202¯1) surfaces relative to c-plane, while reduced indium incorporation was observed on (112¯2) for co-loaded conditions. Indium incorporation was observed to increase with decreasing growth temperature for all planes, while being relatively unaffected by the group III flux rates for a 1:1 Ga:In ratio. Indium incorporation was found to increase at the expense of a decreased growth rate for higher ammonia flows; however, smooth surface morphology was consistently observed for growth on semipolar orientations. Increased concentrations of oxygen and hydrogen were observed on semipolar and nonpolar orientations with a clear trend of increased hydrogen incorporation with indium content.

Effect of Mg codoping on Eu3+ luminescence in GaN grown by ammonia molecular beam epitaxy

The effect of Mg codoping on the Eu3+ luminescence in GaN was investigated by photoluminescence measurements. Two notable emission peaks associated with the 5D0→7F2 transition in the Eu3+ ions were governed by Mg codopants, which corresponded to the change of the dominant peak wavelength from 622.3 to 620.3 nm with an increase in Mg concentration. An optimal amount of Mg also led to enhancement of approximately 20 times of the Eu3+ luminescence. These results indicated that the Mg codopants selectively activated the optical site of 620.3 nm emission due to the elimination of nonradiative deexcitation paths from the 5D0 state.

TEM study of defects in Al xGa 1− xN layers with different polarity

Transmission electron microscopy (TEM) studies of defects in Al x Ga 1− x N layers with various Al mole fractions ( x=0.2, 0.4) and polarities were carried out. The samples were grown by ammonia molecular beam epitaxy on sapphire substrates and consisted of low-temperature AlN (LT-AlN) and high-temperature AlN (HT-AlN) buffer layers, a complex AlN/AlGaN superlattice (SL) and an Al x Ga 1− x N layer ( x=0.2, 0.4). It was observed that at the first growth stages a very high density of dislocations is introduced in both Al-polar and N-polar structures. Then, at the interface of the LT-AlN and HT-AlN layers half-loops are formed and the dislocation density considerably decreases in Al-polar structures, whereas in the N-polar structures such a behavior was not observed. The AlN/AlGaN superlattice efficiently promotes the bend and annihilation of threading dislocations and respectively the decrease of the dislocation density in the upper Al x Ga 1− x N layer with both polarities. The lattice relaxation of metal-polar Al 0.2Ga 0.8N was observed, while N-polar Al 0.2Ga 0.8N did not relax. The dislocation densities in the N-polar Al 0.2Ga 0.8N and Al 0.4Ga 0.6N layers were 5.5×10 9 cm −2 and 9×10 9 cm −2, respectively, and in metal-polar Al 0.2Ga 0.8N and Al 0.4Ga 0.6N layers these were 1×10 10 cm −2 and 6×10 9 cm −2, respectively. Moreover, from TEM images the presence of inversion domains (IDs) in N-polar structures has been observed. The widths of IDs varied from 10 to 30 nm. Some of the IDs widen during the growth of the AlN buffer layers. The IDs formed hills on the surface of the N-polar structures.

AlGaN/GaN heterojunction bipolar transistors by ammonia molecular beam epitaxy

AbstractWe demonstrate AlGaN/GaN heterojunction bipolar transistors (HBTs) by ammonia molecular beam epitaxy (NH3 MBE). The several benefits offered by NH3 MBE for the growth of GaN based vertical electronic devices are discussed. To obtain good ohmic contacts to the base layer, devices were fabricated with two different processes involving regrowths – regrown base contact (Wafer A) and regrown emitter (Wafer B). Devices on Wafers A and B had similar DC performance. Common emitter (CE) operation with a peak collector current density, JC > 1 kA/cm2 was obtained from devices on both wafers. In CE operation, a current gain (β) of 1.5–3 is observed in devices on both wafers. High base resistance is found to limit the current gain in CE operation, by limiting the base‐emitter bias voltage in the active region of the device. Maximum β values observed from Gummel plots at a fixed base‐collector (BC) bias of zero volts, are β ∼ 15 at JC = 0.88 kA/cm2 for Wafer A and β ∼13.5 at JC = 3 kA/cm2 for Wafer B. In devices on both wafers, variation is observed in the plots of β versus IC (collector current) for devices with the same emitter dimensions and geometry but with different lateral spacing between emitter mesa and base contacts (WEB). Peak β obtained is also observed to vary with WEB. An explanation is provided for the observed behaviour. This work demonstrates the potential of NH3 MBE for the growth of GaN HBTs.

On the polarity of GaN micro- and nanowires epitaxially grown on sapphire (0001) and Si(111) substrates by metal organic vapor phase epitaxy and ammonia-molecular beam epitaxy

The polarity of GaN micro- and nanowires grown epitaxially by metal organic vapor phase epitaxy on sapphire substrates and by molecular-beam epitaxy, using ammonia as a nitrogen source, on sapphire and silicon substrates has been investigated. On Al2O3(0001), whatever the growth technique employed, the GaN wires show a mixture of Ga and N polarities. On Si(111), the wires grown by ammonia-molecular beam epitaxy are almost entirely Ga-polar (around 90%) and do not show inversion domains. These results can be understood in terms of the growth conditions employed during the nucleation stage.

Growth kinetics and electronic properties of unintentionally doped semi-insulating GaN on SiC and high-resistivity GaN on sapphire grown by ammonia molecular-beam epitaxy

Growth of unintentionally doped (UID) semi-insulating GaN on SiC and highly resistive GaN on sapphire using the ammonia molecular-beam epitaxy technique is reported. The semi-insulating UID GaN on SiC shows room temperature (RT) resistivity of 1011 Ω cm and well defined activation energy of 1.0 eV. The balance of compensation of unintentional donors and acceptors is such that the Fermi level is lowered to midgap, and controlled by a 1.0 eV deep level defect, which is thought to be related to the nitrogen antisite NGa, similar to the “EL2” center (arsenic antisite) in unintentionally doped semi-insulating GaAs. The highly resistive GaN on sapphire shows RT resistivity in range of 106–109 Ω cm and activation energy varying from 0.25 to 0.9 eV. In this case, the compensation of shallow donors is incomplete, and the Fermi level is controlled by levels shallower than the 1.0 eV deep centers. The growth mechanisms for the resistive UID GaN materials were investigated by experimental studies of the surface kinetics during growth. The required growth regime involves a moderate growth temperature range of 740–780 °C, and a high ammonia flux (beam equivalent pressure of 1×10−4 Torr), which ensures supersaturated coverage of surface adsorption sites with NHx radicals. Such highly nitrogen rich growth conditions lead to two-dimensional layer by layer growth and reduced oxygen incorporation.

Features of molecular beam epitaxy of the GaN (0001) and GaN (000 % MathType!MTEF!2!1!+- % feaagaart1ev2aaatCvAUfKttLearuqr1ngBPrgarmWu51MyVXguY9 % gCGievaerbd9wDYLwzYbWexLMBbXgBcf2CPn2qVrwzqf2zLnharyav % P1wzZbItLDhis9wBH5garqqtubsr4rNCHbGeaGqiVu0Je9sqqrpepC % 0xbbL8F4rqqrFfpeea0xe9Lq-Jc9vqaqpepm0xbba9pwe9Q8fs0-yq % aqpepae9pg0FirpepeKkFr0xfr-xfr-xb9adbaqaaeGaciGaaiaabe % qaamaaeaqbaaGcbaGafGymaeJbaebaaaa!3CDF! $$ \bar 1 $$ ) layers with the use of different methods of activation of nitrogen

The results of comparative studies of the growth kinetics of the GaN layers of different polarity during ammonia molecular beam epitaxy and plasma-assisted molecular beam epitaxy (PA MBE) of nitrogen with the use of sapphire substrates and GaN(000\( \bar 1 \))/c-Al2O3 templates grown by gas-phase epitaxy from metalorganic compounds are presented. The possibility is shown of obtaining the GaN layers with an atomically smooth surface during molecular beam epitaxy with plasma activation of nitrogen. For this purpose, it is suggested to carry out the growth in conditions enriched with metal near the mode of formation of the Ga drops at a temperature close to the decomposition temperature of GaN (TS ≈ 760°C). The conclusion is made that an increase in the growth temperature positively affects the structural, optical, and electrical properties of the GaN (000\( \bar 1 \)) layers. A high quality of the GaN (0001) films grown by the PA MBE method at a low temperature of ∼700°C on the GaN/c-Al2O3 templates is shown.

Different pressure behavior of GaN/AlGaN quantum structures grown along polar and nonpolar crystallographic directions

High quality GaN/AlGaN multiquantum well (QW) structures were grown by ammonia molecular beam epitaxy along the (0001) polar and (112¯0) nonpolar directions. Each sample contains three QWs with thicknesses of 2, 3, and 4 nm as well as 10 nm Al0.30Ga0.70N barriers. The measured photoluminescence (PL) spectrum consists of three peaks originating from the radiative recombination of excitons in individual QWs. In the nonpolar sample, the energy positions (EPL) of the observed peaks are separated because of the quantum confinement effect, whereas in the polar sample an additional redshift is induced by the quantum confined Stark effect. The dependence of EPL on QW width was used to estimate the built-in electric field magnitude in the latter sample to be about 2 MV/cm. Hydrostatic pressure studies of the PL in both samples gave qualitatively different results. In the polar sample, the pressure shift of EPL, dEPL/dp decreases significantly with QW width. The important finding is derived from the observation of a QW width independent dEPL/dp in the nonpolar sample. It shows that for GaN/Al0.30Ga0.70N, the quantum confinement remains practically independent of the applied hydrostatic pressure. This result reveals that in the polar sample, the variation in dEPL/dp with the QW width is due to the pressure-induced increase in the built-in electric field Fint. Thus, a more quantitative analysis of the latter effect becomes justified. We found that the Fint increases with pressure with a rate of about 80 kV(cm GPa)−1.

Selectively grown AlGaN/GaN HEMTs on Si(1 1 1) substrates for integration with silicon microelectronics

The possibility of fabricating AlGaN/GaN high electron mobility transistors from selectively grown mesas as an approach towards the integration of nitrides with silicon microelectronics is reported. The growth process uses a single 100-nm-thick AlN buffer layer for maskless selective growth by ammonia molecular beam epitaxy. The AlN buffer layer is locally etched back except at the areas where the regrowth is required. Sawtooth-shaped AlN mesa patterning, forming facets that are aligned 30°-off the Si[1 1 2¯] direction, are used to obtain smooth GaN{1 0 1¯ 2} facets. A total thickness of about 1.2 μm was sufficient to obtain crack-free, AlGaN/GaN high electron mobility transistors (HEMTs) of high electrical quality. The fabricated devices exhibited maximum drain current and transconductance of 1.1 A/mm and 225 mS/mm, respectively. The growth process reported in this study offers the possibility to grow high-quality materials while leaving a large area of the silicon substrate free for possible integration with silicon microelectronics.

Mechanisms of ammonia—MBE growth of GaN on SiC for transport devices

The effect of growth parameters on impurity incorporation, surface morphology, crystal quality, and the strain state has been studied for ammonia molecular beam epitaxy (MBE) growth of GaN on SiC substrates. Study of the growth mechanisms has identified specific growth regimes ideal for growth of high quality and low strain, crack-free GaN/AlGaN HFETs on SiC wafers. A 2-D growth regime was found at moderate temperatures around 780 °C and high ammonia flow of 200 sccm, which yields smooth interface and low oxygen impurity level, but quite high tensile strain. Combined with a strain relief GaN/AlN (0.2 μm/0.2 μm) buffer structure, this approach produced excellent HFETs with mobilities of 1700–1800 cm 2/V s and near zero or compressive strain. A 3-D growth regime was found at high temperatures around 880 °C and moderate ammonia flow of 100 sccm. Although 3-D growth results in rougher surface, this approach yields low tensile strain (<0.1%) and does not require multilayered strain relief structure. Electron mobility exceeding 1600 cm 2/V s has been achieved using the 3-D growth approach.

Deep level optical and thermal spectroscopy of traps in n-GaN grown by ammonia molecular beam epitaxy

The incorporation of deep level defects in n-type GaN grown by ammonia-based molecular beam epitaxy (MBE) is studied via systematic adjustment of the NH3/Ga flux ratio. Deep level optical and transient spectroscopies, which together enable deep level detection throughout the GaN bandgap, reveal defect states whose individual concentrations vary with the NH3/Ga flux ratio. A general trend of lower concentration for deep levels at EC−3.28, EC−1.28, EC−0.62, and EC−0.25 eV with higher NH3/Ga flux ratio was observed, with the strongest reduction at the EC−0.25 eV level, consistent with expectations for a VN-related defect. The known CN impurity state at EC−3.28 eV and suspected CI-related state at EC−1.28 eV also showed a moderate decrease in concentration at the higher NH3/Ga flux ratio. In contrast, the VGa-related defect at EC−2.62 eV was insensitive to the NH3/Ga flux ratio over the range studied here. Taken together, ammonia-MBE GaN has deep level defects with different sensitivities in flux ratios suggestive of independent physical sources. However, the total trap concentrations were significantly reduced for higher NH3/Ga flux ratios in n-type GaN grown by ammonia-MBE under the range of growth conditions used in this study, suggesting that higher NH3/Ga flux ratios will generate higher electronic quality GaN material when using ammonia-based MBE for device applications.

Structural and morphological properties of GaN buffer layers grown by ammonia molecular beam epitaxy on SiC substrates for AlGaN/GaN high electron mobility transistors

The impact of growth conditions on the surface morphology and structural properties of ammonia molecular beam epitaxy GaN buffers layers on SiC substrates was investigated. The threading dislocation (TD) density was found to decrease with decreasing NH3:Ga flux ratio, which corresponded to an increase in surface roughness and reduction in residual compressive lattice mismatch stress. Furthermore, the dislocation density and compressive stress decreased for increasing buffer thickness. TD inclination was proposed to account for these observations. Optimized surface morphologies were realized at high NH3:Ga flux ratios and were characterized by monolayer-high steps, spiral hillocks, and pyramidal mounds, with rms roughness of ∼1.0 nm over 2×2 μm2 atomic force microscopy images. Smooth surface morphologies were realized over a large range of growth temperatures and fluxes, and growth rates of up to 1 μm/h were achieved. TD densities in the buffers as low as 3×109 cm−2 were demonstrated. These buffers were highly insulating and were used in recently reported AlGaN/GaN HEMTs with power densities of &amp;gt;11 W/mm at 4 and 10 GHz.

X- and Ka-band power performance of AlGaN/GaN HEMTs grown by ammonia-MBE

A report is presented on the power performance of deep submicron AIGaN/GaN high electron mobility transistors grown by ammonia molecular beam epitaxy. At 10 GHz, 70% power-added-efficiency (PAE) and 3 W/mm power density were demonstrated at a drain bias of 20 V. At 30 GHz, 31% PAE and 6.5 W/mm power density were achieved at a drain bias of 40 V.

Power Performance of AlGaN/GaN HEMTs Grown on SiC by Ammonia-MBE at 4 and 10 GHz

In this letter, we report on the microwave power and efficiency performance of AlGaN/GaN high-electron mobility transistors (HEMTs) grown by ammonia molecular beam epitaxy (ammonia-MBE) on SiC substrates. At 4 GHz, an output power density of 11.1 W/mm with an associated power-added efficiency (PAE) of 63% was measured at <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ds</sub> = 48 V on passivated devices. At 10 GHz, an output power density of 11.2 W/mm with a PAE of 58% was achieved for <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ds</sub> = 48 V. These results are the highest reported power performance for AlGaN/GaN HEMTs grown by ammonia-MBE and the first reported for ammonia-MBE on SiC substrates.

Continuous order‐disorder phase transition (2×2)→(1×1) on the (0001)AlN surface

AbstractSmooth (0001)AlN surface on a (0001) sapphire substrate was prepared by ammonia Molecular Beam Epitaxy. A continuous (2×2)→(1×1) reconstruction phase transition on the (0001)AlN surface under ammonia flux was experimentally investigated by RHEED. The intensity of a fractional (0 1/2) streak was studied as a function of the sample temperature under different ammonia fluxes. The temperature of the transition increases with increasing of the ammonia flux (an effective activation energy is about 1.3 eV). The phase transition is described in the frame of the Bethe‐Peierls approximation. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

GaN nanostructures and HFET structures selectively grown on silicon substrates by ammonia-MBE

Silicon substrate can be used as a highly effective pseudomask to perform selective growth of GaN by ammonia molecular beam epitaxy (MBE). This can be exploited to fabricate novel photonic and electronic device structures on Si substrates. Growth of GaN micro-pyramids as templates for fabricating InGaN quantum dots, and growth of AlGaN/GaN HFET mesa patterns using the selective growth approach is reported. The facet growth characteristics of the selectively grown GaN micro-pyramids are investigated. μ-photoluminescence (μ-PL) measurements resolving emissions from InGaN quantum dots and/or wells on a single GaN pyramid have been performed, but have found no evidence of formation of InGaN single quantum dot at the apex of the GaN pyramid. Possible improved growth approaches for achieving such single quantum dot structures are discussed. Success in selective growth of the HFET mesa pattern and especially the insulating carbon-doped GaN buffer layer is achieved, demonstrating a promising approach for potential integration of nitride devices with silicon circuits.

AlGaN/GaN HFETs fabricated from maskless selectively grown mesas on Si(111) substrates

AlGaN/GaN heterostructure field effect transistors (HFETs) fabricated from maskless selectively grown mesas by ammonia molecular beam epitaxy on Si(111) substrates are demonstrated. With 0.8 μm gate length, the devices exhibited maximum drain current and transconductance of 425 mA/mm and 140 mS/mm, respectively. Values of f T and f MAX of 8 and 19 GHz, respectively, were obtained from RF measurements. A 50 nm-thick SiN x passivation layer was found to be very effective in mitigating the current collapse.