N-type AlxGa1 Research Articles

Preparation of degenerate n-type AlxGa1−xN (0 < x ≤ 0.81) with record low resistivity by pulsed sputtering deposition

Highly conductive AlGaN alloys hold a great technological potential, wherein the degenerate n-type doping is key in reducing parasitic resistances in electronic and opto-electronic devices. Nonetheless, AlxGa1−xN elaborated in conventional methods exhibits degraded electron concentration at high Al compositions. Here, we demonstrated the growth of degenerate n-type AlxGa1−xN (0 < x ≤ 0.81) with record high electron concentrations by using the pulsed sputtering deposition (PSD) method. The fabricated samples were investigated by cathodoluminescence and Hall-effect measurements in studying the related deep-level emission and electron mobility, respectively. Our findings showed that the PSD-grown heavily Si-doped AlxGa1−xN exhibits record low resistivity values: 2.5 × 10−3 Ω cm with an electron concentration of 1.2 × 1020 cm−3 for x = 0.60 and 1.7 × 10−2 Ω cm with an electron concentration of 3.2 × 1019 cm−3 for x = 0.81. Temperature-dependent Hall-effect measurements confirmed the degenerate nature of heavily Si-doped AlxGa1−xN up to x = 0.81. These results clearly show that the PSD n-type AlGaN is quite promising for reducing parasitic resistance in AlGaN based far UVC light-emitting devices and high breakdown voltage electron devices.

Low-threshold voltage ultraviolet light-emitting diodes based on (Al,Ga)N metal–insulator–semiconductor structures

Al-rich III–nitride-based deep-ultraviolet (UV) (275–320 nm) light-emitting diodes are plagued with a low emission efficiency and high turn-on voltages. We report Al-rich (Al,Ga)N metal–insulator–semiconductor UV light-emitting Schottky diodes with low turn-on voltages of <3 V, which are about half those of typical (Al,Ga)N p–i–n diodes. Our devices use a thin AlN film as the insulator and an n-type Al0.58Ga0.42N film as the semiconductor. To improve the efficiency, we inserted a GaN quantum-well structure between the AlN insulator and the n-type AlxGa1−xN semiconductor. The benefits of the quantum-well structure include the potential to tune the emission wavelength and the capability to confine carriers for more efficient radiative recombination.

Al x Ga1 − x As/GaAs(100) hetermostructures with anomalously high carrier mobility

Structural and spectroscopic methods are used to study the epitaxial layers of n-type AlxGa1 − xAs solid solutions produced by the metal-organic chemical vapor deposition method. It is shown that, when AlxGa1 − xAs solid solutions are doped with carbon to a level of (1.2–6.7) × 1017 cm−3, the electron mobility is anomalously high for the given impurity concentration and twice exceeds the calculated value. It is assumed that the ordered arrangement of carbon in the metal sublattice of the solid solution leads to a change in the average distance between impurity ions, i.e., to an increase in the mean free path of the carriers and, consequently, in the carrier mobility. The observed effect has immediate practical importance in the search for various technological ways of increasing the operating speed of functional elements of modern optoelectronic devices. The effect of the anomalously high carrier mobility in the epitaxial layer of a heteropair opens up new opportunities for the development of new structures on the basis of AlxGa1 − xAs compounds.

Heterojunction solar cell with 6% efficiency based on an n-type aluminum–gallium–oxide thin film and p-type sodium-doped Cu2O sheet

In this paper, we describe efforts to enhance the efficiency of Cu2O-based heterojunction solar cells fabricated with an aluminum–gallium–oxide (Al–Ga–O) thin film as the n-type layer and a p-type sodium (Na)-doped Cu2O (Cu2O:Na) sheet prepared by thermally oxidizing copper sheets. The optimal Al content [X; Al/(Ga + Al) atomic ratio] of an AlX–Ga1−X–O thin-film n-type layer was found to be approximately 2.5 at. %. The optimized resistivity was approximately 15 Ω cm for n-type AlX–Ga1−X–O/p-type Cu2O:Na heterojunction solar cells. A MgF2/AZO/Al0.025–Ga0.975–O/Cu2O:Na heterojunction solar cell with 6.1% efficiency was fabricated using a 60-nm-thick n-type oxide thin-film layer and a 0.2-mm-thick Cu2O:Na sheet with the optimized resistivity.

Localized deep levels in AlxGa1−xN epitaxial films with various Al compositions

By using high-temperature deep-level transient spectroscopy (HT-DLTS) and other electrical measurement techniques, localized deep levels in n-type AlxGa1−xN epitaxial films with various Al compositions (x = 0, 0.14, 0.24, 0.33, and 0.43) have been investigated. It is found that there are three distinct deep levels in AlxGa1−xN films, whose level position with respect to the conduction band increases as Al composition increases. The dominant defect level with the activation energy deeper than 1.0 eV below the conduction band closely follows the Fermi level stabilization energy, indicating that its origin may be related to the defect complex, including the anti-site defects and divacancies in AlxGa1−xN films.

Electrical properties and microstructure of vanadium-based contacts on ICP plasma etched n-type AlGaN:Si and GaN:Si surfaces

Light-emitting diodes emitting in the UV-B spectral range usually contain an n-type AlxGa1–xN contact layer with x of about 0.4 and require a low specific contact resistivity in the range of 10−6 Ω cm2 to operate at their optimal capacities. We compare the Ti/Al/Mo/Au metal system commonly used on GaN with the V/Al//V/Au metal system on n-Al0.4Ga0.6N:Si and GaN:Si surfaces. On Al0.4Ga0.6N, the lowest specific contact resistivity, (2.3 ± 0.2) ×10−6 Ω cm2, was reached with V/Al/V/Au whereas for Ti/Al/Mo/Au the lowest resistivity was (1.7 ± 0.7) ×10−3 Ω cm2. Independently of the metal system, lower contact resistivities are obtained on GaN where Ti/Al/Mo/Au is still performing at least one order of magnitude better than V/Al/V/Au. The contact resistivity of V/Al/V/Au on Al0.4Ga0.6N was found to be sensitive to surface treatments (wet chemical etching or sputtering) applied prior to metal deposition, to the thickness of the first vanadium layer and to that of the aluminum layer above. Structural and compositional investigations showed that a low contact resistivity is accompanied by the formation of an interfacial layer between AlGaN and V/Al/V/Au on the nm-scale, which contains aluminum, in the form of aluminum nitride, and separately vanadium as metal or metal nitride.

Effect of thermal annealing on ohmic contacts properties of undoped and Si-doped AlxGa1−xN on Si (1 1 1) substrate grown by PA-MBE

In this work, we study the ohmic contact properties of titanium (Ti)/aluminum (Al) bi-layer contacts on undoped and n-type doped AlxGa1−xN grown on silicon (111) substrates by radio frequency nitrogen plasma-assisted molecular beam epitaxy (PA-MBE). The electrical stability of the contacts at various annealing temperatures of 400, 500, 600 and 700°C were investigated. Specific contact resistivity was determined using transmission line method (TLM) and current–voltage (I–V) measurements. The results reveal that the bi-layer scheme was sensitive to the change of annealing temperatures and annealing time. The optimal value of specific contact resistivities was obtained at annealing temperature of 600°C for both samples. However, the values of n-type doped sample exhibited better results compared with the undoped sample.

Observation of longitudinal-optic-phonon-plasmon-coupled mode in n-type AlGaN alloy films

An longitudinal-optic-phonon-plasmon coupled mode (LOPC) has been clearly observed by Raman scattering in n-type AlxGa1−xN films with x ∼ 0.67 and different carrier densities n = 1 × 1017-9 × 1017 cm−3. The A1-LO-phonon mode showed a systematic frequency shift and broadening with increasing n. This is a characteristic behavior of LOPC as previously observed in n-type binary semiconductors. A theoretical line-shape fitting analysis was conducted for the LOPC profile using n and plasmon-damping rate as adjustable parameter. Assuming m*/m0 = 0.28 for the longitudinal effective mass of electron, the analysis well reproduced carrier density and mobility deduced by Hall measurement.

GaN detector development for particle and X-ray detection

We report on preliminary alpha particle and X-ray measurements on a number of prototype GaN PIN diodes. The aim of the study was to investigate the potential use of GAN based radiation detectors for radiation hard, high temperature, solar blind space applications. The devices have a planar structure consisting of a 2μm epitaxial GaN layer grown on a highly doped n-type AlxGa1−xN nucleation layer, which in turn is deposited on a p-type 4H–SiC substrate. Au ohmic contacts were applied to the top of the GaN layer and the bottom of the substrate. A number of different sized devices were tested with contact diameters ranging from 0.4mm to 0.7mm. All devices showed good diode behaviour with reverse leakage currents in the tens to hundreds of micro-amp range. C–V measurements showed that the GaN layers were fully depleted for biases >20V. When exposed to a 5.5MeV alpha particle source, the devices showed a spectroscopic response with energy resolutions of ∼25% FWHM at room temperature (RT) and 10V bias and 20% FWHM at −50°C. These values are consistent with the previous measurements. No response to 60keV photons could be measured.

Effects of high doping on the bandgap bowing for AlxGa1−xN

This paper gives the composition dependence of the bandgap energy for highly doped n-type AlxGa1−xN. We report results of the bowing parameter obtained using a random simulation. Three groups of AlxGa1−xN semiconductors were considered and which are distinguishable by their non degenerate or degenerate character in the doping density (1017⩽ND⩽1020cm−3). A striking feature is the large discrepancy of the bandgap bowing (−2.02⩽b⩽2.94eV), as was demonstrated from our calculations. This suggests that high doping may be a possible cause able to induce the large range of bowing parameters reported for AlxGa1−xN alloys.

Investigation of The Electrical and Chemical Properties of Plasma-Treated AlGaN

ABSTRACTThe surface properties of n-type AlxGa1−xN (x=0-0.5) exposed to inductively couple plasma were studied systematically using metal contact measurements and X-ray photoelectron spectroscopy (XPS). Cl2/BCl3 and Ar plasma treatment considerably increased the surface conductivity of AlxGa1−xN (x&lt;0.3) and thus improved the characteristics of Ti/Al-based ohmic contacts, but degraded the surface and contact properties of AlxGa1−xN (x≥0.3). XPS measurements revealed a significant increase in oxygen incorporation and energy shifts of the core-level spectra in plasma-treated samples. In contrast to an upward shift of the surface Fermi level in plasma-treated GaN, the surface Fermi level moved away from the conduction band in Al0.5Ga0.5N upon plasma treatment. These findings suggest that the nature of plasma damage in AlGaN appears to be a function of the Al mole fraction. Plasma-induced defects in high-Al AlGaN may act as deep-level compensation centers, degrading the surface electrical properties and contact characteristics.

Transport properties of highly conductive n-type Al-rich AlxGa1−xN(x⩾0.7)

We report here the growth and transport studies of conductive n-type AlxGa1−xN alloys with high Al contents (x⩾0.7). Si-doped AlxGa1−xN alloys were grown by metalorganic chemical vapor deposition on AlN-epilayer∕sapphire substrates with very smooth surface. Low n-type resistivities have been obtained for Al-rich AlxGa1−xN alloys. The resistivity was observed to increase rapidly with increasing x due to the deepening of the Si donor energy level. Transport measurements have indicated that we have achieved n-type conduction in pure AlN. From the temperature dependence of the resistivity, the donor activation energy was estimated to vary from 23to180meV as x was increased from 0.7 to 1.0.

Experimental study of perpendicular transport in weakly coupled AlxGa1−xN/GaN superlattices

Perpendicular transport characteristics of n-type AlxGa1−xN/GaN superlattices are presented. Planar and mesa-etched superlattice structures are employed to identify the perpendicular resistance. Perpendicular transport measurements in Al0.22Ga0.78N/GaN superlattices display linear current–voltage characteristics with a resistivity that is a factor of 6.6 higher than for bulk material. A theoretical model is developed for perpendicular transport in AlxGa1−xN/GaN superlattices based on sequential tunneling. The model shows that short superlattice periods are required to minimize the perpendicular resistivity.

Characterisation of iridium Schottky contacts on n -Al x Ga 1− x N

The electrical characteristics of Ir Schottky contacts on n-type AlxGa1−xN (x=0, 0.10, 0.20, 0.25) grown by MOCVD on sapphire substrate are investigated. The barrier heights are obtained from current–voltage (I–V) and capacitance–voltage (C–V) measurements and are found to increase with aluminium composition. The effect of thermal annealing on barrier height of Ir Schottky diodes on Al0.25Ga0.75N is also studied. Only slight degradation in Schottky barrier height is observed after annealing the Ir-Schottky diodes at 850°C for 30 s.

Thermal stability of rhenium Schottky contacts on n-type AlxGa1−xN

The impact of rapid thermal annealing on the electrical and materials characteristics of Re Schottky contacts on n-type GaN and AlxGa1−xN (x=0.10 and 0.26) was investigated. Effective barrier heights were obtained from current–voltage and modified Norde measurements on diodes annealed at up to 800 °C. For AlxGa1−xN with x&amp;gt;0, Schottky barrier heights were found to increase upon annealing from the as-deposited state, but decreased sharply after annealing at 600 °C. Modified Norde measurements indicate that this degradation could be explained by the existence of a shunt conduction path with an associated barrier height below 0.46 V, possibly a consequence of an inhomogeneous interface after annealing at temperatures above 600 °C. A new defect lying at 0.34 eV below the conduction band edge was also detected by deep level transient spectroscopy after contact degradation.

Intentional control of n-type conduction for Si-doped AlN and AlXGa1−XN (0.42⩽x&amp;lt;1)

We have obtained n-type conductive Si-doped AlN and AlXGa1−XN with high Al content (0.42⩽x&amp;lt;1) in metalorganic vapor phase epitaxy by intentionally controlling the Si dopant density, [Si]. Si-doped AlN showed the n-type conduction when [Si] was less than 3×1019 cm−3. When [Si] was more than 3×1019 cm−3, it became highly resistive due to the self-compensation of Si donors. This indicates that the self-compensation plays an important role at higher [Si] and determines the upper doping limit of Si for the AlN and AlXGa1−XN. For x⩾0.49, the ionization energy of Si donors increased sharply with increasing Al content. These resulted in a sharp decrease in the highest obtainable electron concentration with increasing Al content for the Si-doped AlXGa1−XN.

Achieving highly conductive AlGaN alloys with high Al contents

Si-doped n-type AlxGa1−xN alloys were grown by metalorganic chemical vapor deposition on sapphire substrates. We have achieved highly conductive n-type AlxGa1−xN alloys for x up to 0.7. A conductivity (resistivity) value of 6.7 Ω−1 cm−1 (0.15 Ω cm) (with free electron concentration 2.1×1018 cm−3 and mobility of 20 cm2/Vs at room temperature) has been achieved for Al0.65Ga0.35N, as confirmed by Hall-effect measurements. Our experimental results also revealed that (i) the conductivity of AlxGa1−xN alloys continuously increases with an increase of Si doping level for a fixed value of Al content and (ii) there exists a critical Si-dopant concentration of about 1×1018 cm−3 that is needed to convert insulating AlxGa1−xN with high Al content (x⩾0.4) to n-type.

Electrical isolation of AlxGa1−xAs by ion irradiation

The evolution of sheet resistance Rs of n-type and p-type conductive AlxGa1−xAs layers (x=0.3, 0.6, and 1.0) during proton irradiation was investigated. The threshold dose Dth to convert a conductive layer to a highly resistive one is slightly different for n- and p-type samples with similar initial free carrier concentration and does not depend on the Al content. The thermal stability of the isolation, i.e., the temperature range for which the Rs is maintained at ≈109 Ω/sq, was found to be dependent on the ratio of the carrier trap concentration to the original carrier concentration. The thermal stability of isolated p-type samples is limited to temperatures lower than 450 °C. The temperature of ≈600 °C is the upper limit for the n-type samples thermal stability.

Novel Polarization Enhanced Ohmic Contacts to n-Type GaN

Utilizing a thin n-type AlxGa1—xN layer on top of n-type GaN, the metal–semiconductor contact resistance can be decreased drastically compared to bulk n-type GaN. Specific contact resistances as low as 8.5 × 10—5 Ω cm2 have been achieved without high temperature annealing. The design of this novel contact is based on polarization fields in the thin AlxGa1—xN layer.

Polarization charges and polarization-induced barriers in AlxGa1−xN/GaN and InyGa1−yN/GaN heterostructures

Polarization charges are measured and the formation of large electrostatic barriers arising primarily as a consequence of the presence of polarization-induced charge densities is deduced from capacitance–voltage analysis of n-type AlxGa1−xN/GaN and InyGa1−yN/GaN heterostructures. In structures consisting of 5–10 nm AlxGa1−xN or InyGa1−yN surrounded by n-GaN, capacitance–voltage profiling studies combined with elementary electrostatic analysis yield experimental estimates of polarization charge densities, which are compared with values expected based on the combined effects of spontaneous and piezoelectric polarization. These results imply the existence of electrostatic barriers that are due primarily to the large polarization charge densities at each heterojunction interface and the resulting potential difference maintained across the thin AlxGa1−xN or InyGa1−yN layers. The electrostatic barriers formed in these structures are large in comparison to the heterojunction conduction-band offsets, demonstrating the utility of polarization-based engineering of electrostatic barriers in nitride semiconductor heterostructures.