AlGaN/GaN HEMTs grown on silicon (001) substrates by molecular beam epitaxy

We report the realization of an AlGaN/GaN HEMT on silicon (001) substrate with noticeably better transport and electrical characteristics than previously reported. The heterostructure has been grown by molecular beam epitaxy. The 2D electron gas formed at the AlGaN/GaN interface exhibits a sheet carrier density of 8×10¹² cm⁻² and a Hall mobility of 1800 cm²/V s at room temperature. High electron mobility transistors with a gate length of 4 μm have been processed and DC characteristics have been achieved. A maximum drain current of more than 500 mA/mm and a transconductance g_m of 120 mS/mm have been obtained. These results are promising and open the way for making efficient AlGaN/GaN HEMT devices on Si(001).     

In-plane gate transistors in AlxGa1−xN/GaN heterostructures written by focused ion beams

Focused ion beam implantation of gallium and dysprosium was used to locally insulate the near-surface two-dimensional electron gas of Al_xGa_1−xN/GaN heterostructures. The threshold dose for insulation was determined to be 2×10¹⁰ cm⁻¹ for 90 keV Ga⁺ and 1×10⁹ cm⁻¹ for 200 keV Dy²⁺ at 4.2 K. This offers a tool not only for inter-device insulation but also for direct device fabrication. Making use of “open-T” like insulating line patterns, in-plane gate transistors have been fabricated by focused ion beam implantation. An exemplar with a geometrical channel width of 1.5 μm shows a conductance of 32 μS at 0 V gate voltage and a transconductance of around 4 μS, which is only slightly dependent on the gate voltage.     

The effects of GaN capping layer thickness on two-dimensional electron mobility in GaN/AlGaN/GaN heterostructures

In this paper we present a study of the effect of GaN capping layer thickness on the two-dimensional (2D)-electron mobility and the two-dimensional electron gas (2DEG) sheet density which is formed near the AlGaN barrier/buffer GaN layer. This study is undertaken using a fully numerical calculation for GaN/Al_xGa_1−xN/GaN heterostructures with different Al mole fraction in the Al_xGa_1−xN barrier, and for various values of barrier layer thickness. The results of our analysis clearly indicate that increasing the GaN capping layer thickness leads to a decrease in the 2DEG density. Furthermore, it is found that the room-temperature 2D-electron mobility reaches a maximum value of approximately 1.8×10³ cm² /Vs⁻¹ for GaN capping layer thickness grater than 100 Å with an Al_0.32Ga_0.68N barrier layer of 200 Å thick. In contrast, for same structure, the 2DEG density decreases monotonically with GaN capping layer thickness, and eventually saturates at approximately 6×10¹² cm⁻² for capping layer thickness greater than 500 Å. A comparison between our calculated results with published experimental data is shown to be in good agreement for GaN capping layers up to 500 Å thickness.     

Correlation between morphological, electrical and optical properties of GaN at all stages of MOVPE Si/N treatment growth

GaN has been grown using Si/N treatment growth by MOVPE on sapphire (0001) in a home-made vertical reactor. The growth was monitored by in situ laser reflectometry. The morphological, electrical and optical properties of GaN are investigated at all the growth stages. To this aim, the growth was interrupted at different stages. The obtained samples are ex situ characterized by scanning electron microscopy (SEM), room temperature Van der Pauw–Hall electrical transport and low temperature (13 K) photoluminescence (PL) measurements. The SEM images show clearly the coalescence process. A smooth surface is obtained for a fully coalesced layer. During the coalescence process, the electron concentration (n) and mobility (μ) vary from 2×10¹⁹ cm⁻³ to 2×10¹⁷ cm⁻³ and 12 cm²/V s–440 cm²/V s, respectively. The PL maxima shift to higher energy and the FWHM decreases to about 4 meV. A correlation between PL spectra and Hall effect measurements is made. We show that the FWHM follows a n^2/3 power law for n above 10¹⁸ cm⁻³.     

The structure of crystallographic damage in GaN formed during rare earth ion implantation with and without an ultrathin AlN capping layer

The crystallographic nature of the damage created in GaN implanted by rare earth ions at 300 keV and room temperature has been investigated by transmission electron microscopy versus the fluence, from 7×10¹³ to 2×10¹⁶ at/cm², using Er, Eu or Tm ions. The density of point defect clusters was seen to increase with the fluence. From about 3×10¹⁵ at/cm², a highly disordered ‘nanocrystalline layer’ (NL) appears on the GaN surface. Its structure exhibits a mixture of voids and misoriented nanocrystallites. Basal stacking faults (BSFs) of I₁, E and I₂ types have been noticed from the lowest fluence, they are I₁ in the majority. Their density increases and saturates when the NL is observed. Many prismatic stacking faults (PSFs) with Drum atomic configuration have been identified. The I₁ BSFs are shown to propagate easily through GaN by folding from basal to prismatic planes thanks to the PSFs.When implanting through a 10 nm AlN cap, the NL threshold goes up to about 3×10¹⁶ at/cm². The AlN cap plays a protective role against the dissociation of the GaN up to the highest fluences. The flat surface after implantation and the absence of SFs in the AlN cap indicate its high resistance to the damage formation.     

Growth and characterisation of GaN with reduced dislocation density

Two GaN MOVPE growth methods to reduce the threading dislocation (TD) density have been explored. The combined effects of (1) in situ SiN_x masking of the sapphire substrate and (2) starting the epitaxial growth at low V-to-III ratio on the GaN film quality were studied by atomic force microscopy, transmission electron microscopy and high-resolution X-ray diffraction. It was found that the annealing condition of the low-temperature nucleation layer after in situ SiN_x masking is critical in order to decrease the density of nucleation sites and hence increase the average grain size to about 5 μm. However, the coalescence of large grains with vertical side facets results in the formation of dense bundles of TDs at the grain boundaries combined with large numbers of basal-plane dislocation loops throughout the film. The formation of these dislocations can be prevented by starting the epilayer growth at low V-to-III ratio, resulting in the formation of grains with inclined side facets. The interaction of the TDs with the inclined side facets causes the dislocations to bend 90 as the grains grow in size and coalesce. GaN films with dislocation densities as low as 1×10⁸ cm⁻², giving full-width at half-maximum values of 180 and 220 arcsec for respectively (002) and (302) omega scans, were achieved by the combination of in situ masking and low V–III ratio epilayer growth. Hall carrier mobility values in excess of 900 cm² V ⁻¹ s⁻¹ were deduced for Si-doped layers.     

Properties of Si-doped GaN and AlGaN/GaN heterostructures grown by RF-MBE on high resistivity Fe-doped GaN

Silicon-doped GaN epilayers and AlGaN/GaN heterostructures were developed by nitrogen plasma-assisted molecular beam epitaxy on high resistivity iron-doped GaN (0001) templates and their properties were investigated by atomic force microscopy, x-ray diffraction and Hall effect measurements. In the case of high electron mobility transistors heterostructures, the AlN mole fraction and the thickness of the AlGaN barrier employed were in the range of from 0.17 to 0.36 and from 7.5 to 30 nm, respectively. All structures were capped with a 2 nm GaN layer.Despite the absence of Ga droplets formation on the surface, growth of both GaN and AlGaN by RF-MBE on the GaN (0001) surfaces followed a step-flow growth mode resulting in low surface roughness and very abrupt heterointerfaces, as revealed by XRD. Reciprocal space maps around the reciprocal space point reveal that the AlGaN barriers are fully coherent with the GaN layer.GaN layers, n-doped with silicon in the range from 10¹⁵ to 10¹⁹ cm⁻³ exhibited state of the art electrical properties, consistent with a low unintentional background doping level and low compensation ratio. The carrier concentration versus silicon cell temperatures followed an Arhenius behaviour in the whole investigated doping range. The degenerate 2DEG, at the AlGaN/GaN heteroiterfaces, exhibited high Hall mobilities reaching 1860 cm²/V s at 300 K and 10 220 cm²/V s at 77 K for a sheet carrier density of 9.6E12 cm⁻².The two dimensional degenerate electron gas concentration in the GaN capped AlGaN/GaN structures was also calculated by self-consistent solving the Schrödinger–Poisson equations. Comparison with the experimental measured values reveals a Fermi level pinning of the GaN (0001) surface at about 0.8 eV below the GaN conduction band.     

Dependences of structural and electrical properties on thickness of polycrystalline Ga-doped ZnO thin films prepared by reactive plasma deposition

Polycrystalline Ga-doped (Ga content: 4 wt%) ZnO (GZO) thin films were deposited on glass substrates at 200 C by a reactive plasma deposition with DC arc discharge technique. The dependences of structural and electrical properties of GZO films on thickness, ranging from 30 to 560 nm, were investigated. Carrier concentration, n, and Hall mobility, μ, increases with increasing film thickness below 100 nm, and then the n remains nearly constant and the μ gradually increases until the thickness reaches 560 nm. The resistivity obtained of the order of 10⁻⁴ Ω cm for these films decreases with increasing film thickness: The highest resistivity achieved is 4.4×10⁻⁴ Ω cm with n of 7.6×10²⁰ cm⁻³ and μ of 18.5 cm²/V s for GZO films with a thickness of 30 nm and the lowest one is 1.8×10⁻⁴ Ω cm with n of 1.1×10²¹ cm⁻³ and μ of 31.7 cm²/V s for the GZO film with a thickness of 560 nm. X-ray diffraction pattern for all the films shows a hexagonal wurtzite structure with its strongly preferred orientation along the c-axis. Full width at half maximum of the (002) preferred orientation diffraction peak of the films decreases with increasing film thickness below 100 nm.     

Low resistance nonalloyed Al-based ohmic contacts on n-ZnO:Al

We have investigated the electrical properties of nonalloyed Al, Al/Au, and Al/Pt ohmic contacts on n-type ZnO:Al (2×10¹⁸ cm⁻³). All Al-based nonalloyed ohmic contacts on the n-ZnO:Al reveal linear current–voltage behavior with low specific contact resistivity of 8.5×10⁻⁴ (Al), 8.0×10⁻⁵ (Al/Au) and 1.2×10⁻⁵ Ω cm² (Al/Pt), respectively. Using secondary ion mass spectroscopy (SIMS) and x-ray photoelectron spectroscopy (XPS) depth profiles, it was found that the O atoms in the ZnO:Al layer outdiffused to Al metal layer while the Al atoms indiffused to the surface region of ZnO:Al. This interdiffusion between Al and O atoms at room temperature results in an increase of doping concentration in the surface region of the ZnO:Al and reduces a specific contact resistivity of the Al-based ohmic contacts without thermal annealing process.     

Influence of AlN interfacial layer on electrical properties of high-Al-content Al0.45Ga0.55N/GaN HEMT structure

Unintentionally doped high-Al-content Al_0.45Ga_0.55N/GaN high electron mobility transistor (HEMT) structures with and without AlN interfacial layer were grown by metal-organic chemical vapor deposition (MOCVD) on two-inch sapphire substrates. The effects of AlN interfacial layer on the electrical properties were investigated. At 300 K, high two-dimensional electron gas (2DEG) density of 1.66 × 10¹³ cm⁻² and high electron mobility of 1346 cm² V⁻¹ s⁻¹ were obtained for the high Al content HEMT structure with a 1 nm AlN interfacial layer, consistent with the low average sheet resistance of 287 Ω/sq. The comparison of HEMT wafers with and without AlN interfacial layer shows that high Al content AlGaN/AlN/GaN heterostructures are potential in improving the electrical properties of HEMT structures and the device performances.     

MBE growth of nitride-based photovoltaic intersubband detectors

In this work, we present the plasma-assisted molecular-beam epitaxial growth of quantum well infrared photodetector (QWIP) structures, including the Si-doped GaN/AlN short-period superlattice of the active region, conductive AlGaN claddings and integration of the final device. The growth of Si-doped GaN/AlN multiple quantum well (QW) structures is optimized by controlling substrate temperature, metal excess and growth interruptions. Structural characterization confirms a reduction of the interface roughness to the monolayer scale. P-polarized intersubband absorption peaks covering the 1.33–1.91 μm wavelength range are measured on samples with QW thickness varying from 1 to 2.5 nm. The absorption exhibits Lorentzian shape with a line width around 100 meV in QWs doped 5×10¹⁹ cm⁻³. To prevent partial depletion of the QWs owing to the internal electric field, we have developed highly-conductive Si-doped AlGaN cladding layers using In as a surfactant during growth. Complete ISB photodetectors with 40 periods of 1 nm-thick Si-doped GaN QWs with 2 nm-thick AlN barriers have been grown on conductive AlGaN claddings, the Al mole fraction of the cladding matching the average Al content of the active region. Temperature-dependent photovoltage measurements reveal a narrow (90 meV) detection peak at 1.39 μm.     

Structural properties of 10 μm thick InN grown on sapphire (0001)

The structural properties of a 10 μm thick In-face InN film, grown on Al₂O₃ (0001) by radio-frequency plasma-assisted molecular beam epitaxy, were investigated by transmission electron microscopy and high resolution x-ray diffraction. Electron microscopy revealed the presence of threading dislocations of edge, screw and mixed type, and the absence of planar defects. The dislocation density near the InN/sapphire interface was 1.55×10¹⁰ cm⁻², 4.82×10⁸ cm⁻² and 1.69×10⁹ cm⁻² for the edge, screw and mixed dislocation types, respectively. Towards the free surface of InN, the density of edge and mixed type dislocations decreased to 4.35×10⁹ cm⁻² and 1.20×10⁹ cm⁻², respectively, while the density of screw dislocations remained constant. Using x-ray diffraction, dislocations with screw component were found to be 1.2×10⁹ cm⁻², in good agreement with the electron microscopy results. Comparing electron microscopy results with x-ray diffraction ones, it is suggested that pure edge dislocations are neither completely randomly distributed nor completely piled up in grain boundaries within the InN film.     

Electrical characterisation of phosphorus-doped ZnO thin films grown by pulsed laser deposition

Phosphorus-doped ZnO films were grown by pulsed laser deposition using a ZnO:P₂O₅-doped target as the phosphorus source with the aim of producing p-type ZnO material. ZnO:P layers (with phosphorus concentrations of between 0.01 to 1 wt%) were grown on a pure ZnO buffer layer. The electrical properties of the films were characterised from temperature dependent Hall-effect measurements. The samples typically showed weak n-type conduction in the dark, with a resistivity of 70 Ω cm, a Hall mobility of μ_n0.5 cm² V ⁻¹ s⁻¹ and a carrier concentration of n3×10¹⁷ cm⁻³ at room temperature. After exposure to an incandescent light source, the samples underwent a change in conduction from n- to p-type, with an increase in mobility and decrease in concentration for temperatures below 300 K.     

Thermal oxidation of n-type ZnN films made by -sputtering from a zinc nitride target, and their conversion into p-type films

Transparent p-type thin films, containing zinc oxide phases, have been fabricated from the oxidation of n-type zinc nitride films. The zinc nitride thin films were deposited by rf-magnetron sputtering from a zinc nitride target in pure N₂ and pure Ar plasma. Films deposited in Ar plasma were conductive (resistivity 4.7×10⁻² Ω cm and carrier concentrations around 10²⁰ cm⁻³) Zn-rich Zn_xN_y films of low transmittance, whereas Zn_xN_y films deposited in N₂ plasma showed high transmittance (>80%), but five orders of magnitude lower conductivity. Thermal oxidation up to 550 C converted all films into p-type materials, exhibiting high resistivity, 10²–10³ Ω cm, and carrier concentration around 10¹³ cm⁻³. However, upon oxidation, the Zn_xN_y films did not show the zinc oxide phase, whereas Zn-rich Zn_xN_y films were converted into films containing ZnO and ZnO₂ phases. All films exhibited transmittance >85% with a characteristic excitonic dip in the transmittance curve at 365 nm. Low temperature photoluminescence revealed the existence of exciton emissions at 3.36 and 3.305 eV for the p-type zinc oxide film.     

Ni–Al ohmic contact to p-type 4H-SiC

Investigations on Ni/Al alloys to form ohmic contacts to p-type 4H-SiC are presented in this paper. Different ratios of Ni/Al were examined. Rapid thermal annealing was performed in argon atmosphere at 400 C for 1 min, followed by an annealing at 1000 C for 2 min. In order to extract the specific contact resistance, TLM test structures were fabricated. A specific contact resistance of 3×10⁻⁵ Ω cm² was obtained reproducibly on Al²⁺ implanted p-type layers, having a doping concentration of 1×10¹⁹ cm⁻³. The lowest specific contact resistance value measured amounts to 8×10⁻⁶ Ω cm².     

The effects of thermal annealing in NH3 -ambient on the p-type ZnO films

Thermal annealing in NH₃-ambient was carried out to form p-type ZnO films. The properties were examined by X-ray diffraction (XRD), Hall-effect measurement, photoluminescence (PL), and secondary ion mass spectrometry (SIMS). Electron concentrations in ZnO films were in the range of 10¹⁵–10¹⁷/cm³ with thermal annealing in NH₃-ambient. The activation thermal annealing process was needed at 800 C under N₂-ambient to obtain p-type ZnO. The electrical properties of the p-type ZnO showed a hole concentration of 1.06×10¹⁶/cm³, a mobility of 15.8 cm²/V s, and a resistivity of 40.18 Ω cm. The N-doped ZnO films showed a strong photoluminescence peak at 3.306 eV at 13 K, which is closely related to neutral acceptor bound excitons of the p-type ZnO. The incorporation of nitrogen was confirmed in the SIMS spectra.     

Scattering mechanisms in (Al, Ga)As/GaAs 2DEG structures

We have prepared a large number of high mobility two-dimensional electron gas (2DEG) structures, with undoped spacer thicknesses ranging from 9 to 3200Å. For samples with 400Å of (Al, Ga)As Si-doped at 1.3×10¹⁸ cm⁻³, there is a peak in the 4K mobility at spacers of 400–800Å, with a maximum value of 2×10⁶ cm² V⁻¹ s⁻¹. Increasing the thickness of the doped (Al, Ga)As to 500Å produced an increase in mobility to 3×10⁶ cm² V⁻¹ s⁻¹ for a 400Å space sample. We have compared these results with published analyses of scattering processes in 2DEG structures, and conclude that a combination of ionised impurity and acoustic phonon scattering gives a qualitative explanation of the behaviour, but that the experimental mobility values are generally higher than those predicted theoretically.     

Carrier concentration and shallow electron states in In-doped hydrothermally grown ZnO

Single-crystal ZnO has been hydrothermally grown with additional In₂O₃ in the solution. Schottky barrier contacts have been deposited by electron beam evaporation of Pd onto the face. Capacitance–voltage measurements have been performed to reveal the carrier concentration as a function of the In₂O₃ content in the solution, and secondary-ion mass spectrometry was used to measure the resulting In concentration in the samples. For an In₂O₃ content of 2×10¹⁹ cm⁻³, the average free electron concentration increased to 5×10¹⁸ cm⁻³ compared to 4×10¹⁷ cm⁻³ for the non-doped material. An increase of the In₂O₃ content to 4×10¹⁹ cm⁻³ leads to a measured carrier concentration of approximately 1×10¹⁹ cm⁻³; however, only up to a quarter of the incorporated In became electrically active. From thermal admittance spectroscopy measurements two prominent electronic levels are found, and compared with to the non-doped material case, the freeze-out of the shallow doping in the In-doped samples takes place at lower temperatures (below 80 K).     

Mobility of electrons and holes in an n-type organic semiconductor perylene diimide thin film

N,N′-diphenylbutyl-3,4,9,10-perylenebiscarboximide (PTCDI-C4Ph) were characterized by optical and electrochemical methods. A device with an ITO/PTCDI-C4Ph (≈2 μm)/Al structure was fabricated to measure mobility by time-of-flight techniques. This vacuum deposited organic layer was an amorphous state. Electrons were observed faster than holes. The electron and hole mobilities were 1.8 × 10⁻⁴ cm²/V s and 1.1 × 10⁻⁴ cm²/V s under the electric field of 500 (V/cm)^1/2, respectively. This result shows that this organic compound is a good candidate for an n-type conduction.     

Characterisation of defects in rare earth implanted GaN by deep level transient spectroscopy

Deep level transient spectroscopy (DLTS) was used to investigate the electrical properties of GaN implanted with the rare earth (RE) ions erbium and thulium. The GaN layers have been grown by metal-organic chemical vapor deposition (MOCVD) onto (0001) sapphire substrates. We used the channeled implantation geometry to implant a dose of 5×10¹⁴ RE cm⁻² with an energy of 150 keV. For each species, two different annealing procedures were used in a nitrogen atmosphere for 120 s. Indeed, the annealing temperature plays an important role in the lattice recovery, even if RE-related defects remain present. After annealing at 1000 C, the appearance of two new peaks, for both studied RE ions, is associated with the lattice damage induced by the implantation, such as the presence of nitrogen vacancies. After annealing at 1100 C, the recovery of the lattice is observed while a hole trap appears for both implanted RE ions with corresponding energy values E_v+0.61 eV and E_v+1.59 eV, in the case of Er and Tm, respectively.