Growth and Annealing Study of Mg-Doped AlGaN and GaN/AlGaN Superlattices

Mg-doped AlGaN and GaN/AlGaN superlattices are grown by metalorganic chemical vapour deposition （MOCVD） Rapid thermal annealing （RTA） treatments are carried out on the samples. Hall and high resolution x-ray diffraction measurements are used to characterize the electrical and structural prosperities of the as-grown and annealed samples, respectively. The results of hall measurements show that after annealing, the Mg-doped AIGaN sample can not obtain the distinct hole concentration and can acquire a resistivity of 1.4 ×10^3 Ωcm. However, with the same annealing treatment, the GaN/AlGaN superlattice sample has a hole concentration of 1.7 × 10^17 cm-3 and a resistivity of 5.6Ωcm. The piezoelectric field in the GaN/AlGaN superlattices improves the activation efficiency of Mg acceptors, which leads to higher hole concentration and lower p-type resistivity.     

Ultraviolet Phototransistors on AlGaN/GaN Heterostructures

We report on the fabrication and characterization of phototransistors based on AIGaN/GaN heterostructure grown over 6H-SiC substrates. The device has two functions： as a high electron mobility transistor （HEMT） and an ultraviolet photodetector at the same time. As an HEMT, its maximum transconductance is 170mS/ram, while the minimum cutoff frequency fT and the maximum oscillation frequency fm are 19 and 35 GHz, respectively. As a photodetector, the device is visible blind, with an ultraviolet/green contrast of three orders of magnitude, and a responsivity as high as 1700 A/W at the wavelength of 362nm.     

Hydrogen Sensors Based on AlGaN/AlN/GaN Schottky Diodes

Pt/AlGaN/AlN/GaN Schottky diodes are fabricated and characterized for hydrogen sensing. The Pt Schottky contact and the Ti/Al/Ni/Au ohmic contact are formed by evaporation. Both the forward and reverse currents of the device increase greatly when exposed to hydrogen gas. A shift of 0.3 V at 300 K is obtained at a fixed forward current after switching from N2 to 10%H2＋N2. The sensor responses under different concentrations from 50ppm H2 to 10%H2＋N2 at 373K are investigated. Time dependences of the device forward current at 0.5 V forward bias in N2 and air atmosphere at 300 and 373K are compared. Oxygen in air azcelerates the desorption of the hydrogen and the recovery of the sensor. Finally, the decrease of the Schottky barrier height and sensitivity Of the sensor are calculated.     

Characterization of Ohmic contacts on GaN/AlGaN heterostructures

The surface morphology and electrical resistance of the contacts on semiconductor devices are strongly influenced by metallization scheme and annealing conditions. In this work is presented an investigation of Ohmic contacts formed by metal-semiconductor alloying on epitaxial GaN/AlGaN heterostructures. After the deposition of metallic multi-layers (Ti, Al, Au and Pt), the process of rapid thermal annealing was carried out in nitrogen, argon and forming gas atmosphere.A series of the samples with different sequences of metallic layers and diverse thicknesses was prepared by employing electron beam evaporation and lift-off deposition techniques. The chemical composition of the samples before and after annealing was studied by means of X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) techniques combined with low energy Ar⁺ ion sputtering. The sputtering has been carried out in two different modes: by using constant (square) or gradually narrowed sputtered area. The changes in the chemical state of constituent elements and compositional profiles of the contacts after thermal annealing were revealed from the obtained results. Among the technological problems, influencing on the quality of the contact, were found to be the oxidation and nitridation of the contact surface during thermal annealing, as well as the intermediate sub-layers of Al and Ti oxides, formed during the deposition of metallic multi-layered structure.     

High-Reflectivity A1GaN/A1N Distributed Bragg Reflector in Ultraviolet Region

Thirty-pair Alo.3 Gao.T N/A1N distributed Bragg reflectors centred at 32Ohm are designed and grown on sapphire substrates by metalorganic chemical vapour deposition. No cracks are observed in the main area of the 2-inch wafer except for about 4 mm margin under an optical microscope. Regular stack of alternating layers is shown by scanning electron microscopy. Clear two-dimensional growth steps and very low surface roughness are shown by atomic force microscopy （AFM）. Well-defined periodicity is shown by high resolution x-ray diffraction. High refiectivity of 93% at 313nm with a bandwidth of 13nm is obtained.     

Electrical Characteristics of InGaN/AlGaN and InGaN/GaN MQW Near UV-LEDs

Electrical characteristics of In0.05 Ga0.95N/Al0.07Ga0.9aN and In0.05 Ga0.95N/GaN multiple quantum well （MQW） ultraviolet light-emltting diodes （UV-LEDs） at 400hm wavelength are measured. It is found that for InGaN/AlGaN MQW LEDs, both ideality factor and parallel resistance are similar to those of InGaN/GaN MQW LEDs, while series resistance is two times larger. It is suggested that the Al0.07Ga0.93N barrier layer did not change crystal quality and electrical characteristic of p-n junction either, but brought larger series resistance. As a result, InGaN/AlGaN MQW LEDs suffer more serious thermal dissipation problem although they show higher light output efficiency.     

Effect of well thickness on the Rashba spin splitting and intersubband spin-orbit coupling in AlGaN/GaN/AlGaN quantum wells with two subbands

By projecting the characteristic equation into the subspace of the conduction band, the Rashba spin splitting coefficient for the first two subbands (α₁, α₂) and the intersubband spin-orbit coupling coefficient (η₁₂) in AlGaN/GaN quantum well structure are obtained. Then sizable α₁, α₂ and η₁₂ in QWs are calculated by solving the Schrödinger and Poisson equations self-consistently. We find that the internal electric field is crucial for considerable spin-orbit coupling effect in III-nitride QWs and the spin-orbit coupling coefficient can be greatly modulated by the well thickness. Compared with the Rashba coefficient, the intersubband spin-orbit coupling coefficient is basically of the same order of magnitude. The results show the great possibility of spin manipulation in low-dimensional semiconductors, and III-nitride QWs are candidates for the design of spintronic devices.     

Improvement of interface properties of AlGaN/GaN heterostructures under gamma-radiation

The cathodoluminescence (CL) spectra of AlGaN/GaN heterostructures grown on sapphire substrate were studied before and after gamma irradiation treatment. The CL spectroscopy results reveal strong yellow and blue luminescence transformation under gamma radiation treatment. The changes in CL spectra are compared with changes in the electrical characteristics of two-dimensional gas in AlGaN/GaN heterostructures. The origins of the observed improvement in properties of AlGaN/GaN heterostructures after gamma radiation treatment with 1 × 10⁶ rad are discussed on the basis of compensation and structural ordering of native defects.     

Back doping design in delta-doped AlGaN/GaN heterostructure field-effect transistors

In this paper, we investigate theoretically the electron transport in AlGaN/GaN single-barrier and in AlGaN/GaN/AlGaN double-barrier heterostructures, aimed to operate as high-power and high-temperature field-effect transistors. The presence of spontaneous and piezoelectric polarizations as well as the heterointerface polarity are evoked and taken into account in the modelling part. Delta-doping is used as a source of electrons for the channel quantum well. Calculations of the electron-band parameters are made by using self-consistent solutions of coupled Schrodinger-Poisson equations. It is found that the polarization fields act to significantly increase the two-dimensional sheet charge concentration. Moreover, the AlGaN/GaN heterostructures with higher Al compositions are found to be favourable for higher electron densities. On the other hand, the employment of a back doping with delta-shaped profiles is shown to improve further the electrical behaviour of the field-effect transistors studied.     

Tunneling induced electron transfer in SiNx/AlGaN/GaN based metal-insulator-semiconductor structures

Tunneling induced electron transfer in SiN_x/Al_0.22Ga_0.78N/GaN based metal-insulator-semiconductor (MIS) structures has been investigated by means of capacitance-voltage (C-V) measurements at various temperatures. Large clock-wise hysteresis window in C-V profiles indicates the injection of electrons from the two-dimensional electron gas (2DEG) channel to the SiN_x layer. Depletion of the 2DEG at positive bias in the negative sweeping direction indicates that the charges injected have a long decay time, which was also observed in the recovery process of the capacitance after injection. The tunneling induced electron transfer effect in SiN_x/Al_0.22Ga_0.78N/GaN based MIS structure opens up a way to design Al_xGa_1−xN/GaN based variable capacitors and memory devices.     

Effect of CO on Characteristics of AlGaN/GaN Schottky Diode

Pt Schottky diode gas sensors for CO are fabricated using A1GaN/GaN high electron mobility transistor（HEMTs）structure. The diodes show a remarkable sensor signal （3 mA, in N2, 2mA in air ambient） biased 2 V after 1% CO is introduced at 50℃. The Schottky barrier heights decrease for 36meV and 27meV in the two cases respectively. The devices exhibit a slow recovery characteristic in air ambient but almost none in the background of pure N2, which reveals that oxygen molecules could accelerate the desorption of CO and offer restrictions to CO detection.     

Neutron Irradiation Effect in Two-Dimensional Electron Gas of AlGaN/GaN Heterostructures

AlGaN/GaN heterostructures have been irradiated by neutrons with different fluences and characterized by means of temperature-dependent Hall measurements and Micro-Raman scattering techniques. It is found that the carrier mobility of two-dimensional electron gas （2DEG） is very sensitive to neutrons. At a low fluence of 6.13 × 10^15 cm^-2, the carrier mobility drops sharply, while the sheet carrier density remains the same as that of an unirradiated sample. Moreover, even for a fluence of up to 3.66 × 10^16 cm^-2, the sheet carrier density shows only a slight drop. We attribute the degradation of the figure-of-merit （product of ns×μ ） of 2DEG to the defects induced by neutron irradiation. Raman measurements show that neutron irradiation does not yield obvious change to the strain state of AlGaN/GaN heterostructures, which proves that degradation of sheet carrier density has no relation to strain relaxation in the present study. The increase of the product of ns × μ of 2DEG during rapid thermal annealing processes at relatively high temperature has been attributed to the activation of GeGa transmuted from Ga and the recovery of displaced defects.     

Beating patterns in the oscillatory magnetoresistance of an AlGaN/GaN heterostructure

Magneto-transport measurements have been carried out on a modulation-doped Al_0.22Ga_0.78N/GaN heterostructure in a temperature range between 1.5 and 25 K with a rather high carrier density, 1.1×10¹³ cm⁻². Striking beating patterns in magnetoresistance vs magnetic field are observed in the vicinity of a special temperature. Theoretical simulation is performed and the comparison between numerical simulations and the experimental data reveals that the beating patterns are due to the interference of the magneto-intersubband scattering and the SdH oscillator of first subband.     

Scattering times in AlGaN/GaN two-dimensional electron gas from magnetotransport measurements

The various scattering times of two-dimensional electron gas were investigated in modulation-doped Al_0.22Ga_0.78N/GaN quantum wells by means of magnetotransport measurements. The ratio of transport and quantum scattering times, τ_t/τ_q∼1, shows that the dominant mobility-limiting mechanisms are short-range scattering potentials. The low-field magnetoresistance shows the weak antilocalization and localization phenomenon from which the spin-orbit scattering and inelastic scattering times are obtained. The inelastic scattering time is found to follow the T⁻¹ law, indicating that electron-electron scattering with small energy transfer is the dominant inelastic process.     

Simulation of polarization effects in AlGaN/GaN heterojunction

Because of their large band-gap, large high-field electron velocity, large breakdownfield, and large thermal conductivity, GaN and its heterojunction with AlGaN and InGaNhave foreseeable potential in the applications of high-power/temperature electronics, andoptoelectronic devices operative in UV and visible wavelength. Polarization inducedelectric field can reach the magnitude of ~MV/cm[1,2]. For AlGaN/GaN based FETs theconcentration of sheet carrier induced by polarization in the cha…     

Giant optical anisotropy in M-plane GaN/AlGaN quantum wells due to crystal-field effect

The optical polarization of GaN/AlGaN wurtzite quantum wells in various orientations is studied using an arbitrarily-oriented [hkil] Hamiltonian potential matrix. The optical matrix elements in the wurtzite quantum wells are calculated using the k⋅p finite difference scheme. The results reveal the presence of giant in-plane optical anisotropy (polarized normal to [0001]) in the M-plane (i.e., the -oriented layer plane) GaN/Al_0.2Ga_0.8N quantum well, due to the positive crystal-field split energy effect (ΔCR>0). The present theoretical results are consistent with the photoluminescence measurements presented in the literature [B. Rau, et al., Appl. Phys. Lett. 77 (2000) 3343].     

Ferroelectric polarization-controlled two-dimensional electron gas in ferroelectric/AlGaN/GaN heterostructure

The control effect of the ferroelectric polarization on the two-dimensional electron gas (2DEG) in a ferroelectric/AlGaN/GaN metal–ferroelectric–semiconductor (MFS) structure is theoretically analyzed by a self-consistent approach. With incorporating the hysteresis nature of the ferroelectric into calculation, the nature of the control effect is disclosed, where the 2DEG density is depleted/restored after poling/depoling operation on the MFS structure. The orientation of the ferroelectric polarization is clarified to be parallel to that of the AlGaN barrier, which, based on an electrostatics analysis, is attributed to the pinning effect of the underlying polarization. Reducing the thickness of the AlGaN barrier from 25 nm to 20 nm leads to an improved control modulation of the 2DEG density from 36.7% to 54.1%.     

Electron mobility in a modulation doped AlGaN/GaN quantum well

We consider a two dimensional electron gas confined to a modulation doped AlGaN/GaN quantum well and study the dependence of low field mobility on various parameters such as composition, well width, remote impurity and interface roughness as a function of temperature. GaN is assumed to be in the zincblende structure. Acoustic and optical phonon, ionized remote impurity and interface roughness scatterings are taken into account in mobility calculations. The scattering rates are calculated using the self-consistently calculated wave functions obtained from the numerical solution of Poisson and Schr?dinger equations. Also found from the self-consistent solutions are the potential profile at the junction, the energy levels in the well and electron concentrations in each level. Ensemble Monte Carlo method is used to find the drift velocities of the two dimensional electrons along the interface under an applied field. The mobility of two dimensional electrons is obtained from the drift velocity of electrons. It is found that while remote impurity scattering is very effective for small values of spacer layer and doping concentrations, increasing Al concentration reduces the mobility of electrons. The effect of surface roughness, on the other hand, on mobility is almost independent of well width. The results of our simulations are compatible with the existing experimental data.     

Giant optical anisotropy in R-plane GaN/AlGaN quantum wells caused by valence band mixing effect

This study investigates the optical anisotropy spectrum in the R-plane (i.e., the -oriented layer plane) of GaN/Al_0.2Ga_0.8N quantum wells of different widths. The optical matrix elements in the wurtzite quantum wells are calculated using the k⋅p finite difference scheme. The calculations show that the valence band mixing effect produces giant in-plane optical anisotropy in -oriented GaN/Al_0.2Ga_0.8N quantum wells with a narrow width. The nature of the in-plane optical anisotropy is found to be dependent on the well width. Specifically, it is found that the anisotropy changes from x^′-polarization to y^′-polarization as the well width increases.     

1.8-kV circular AlGaN/GaN/AlGaN double-heterostructure high electron mobility transistor

In this paper, we present a 1.8-kV circular AlGaN/GaN/AlGaN double-heterostructure high electron mobility transistor(DH HEMT) with a gate-drain spacing L_(GD)= 18.8 μm. Compared with the regular DH HEMT, our circular structure has a high average breakdown electric-field strength that increases from 0.42 MV/cm to 0.96 MV/cm. The power figure of meritV_(BR)~2/RON for the circular HEMT is as high as 1.03 ×10~9 V~2·Ω~(-1)·cm~(-2). The divergence of electric field lines at the gate edge and no edge effect account for the breakdown enhancement capability of the circular structure. Experiments and analysis indicate that the circular structure is an effective method to modulate the electric field.