A. V. Naumov

AlGaN/GaN High Electron Mobility Transistor Structures: Self-Heating Effect and Performance Degradation

This paper reports on the results of the experimental and numerical investigation into the self-heating effect in AlGaN/GaN heterostructures grown on sapphire and SiC substrates. It shows that temperature increase has an opposite dependence on the buffer thickness for sapphire and SiC substrates. Noise spectroscopy is also used to monitor the self-heating effect. Moreover, it is shown that the room-temperature spectra can be used to determine the activation energy of the traps. An irreversible improvement in mobility and quantum scattering time is registered after the irradiation of AlGaN/GaN heterostructures at a total dose of 1 times106 rad of 60Co gamma rays.

Quantum confinement effect on the effective mass in two-dimensional electron gas of AlGaN/GaN heterostructures

We report the results of direct measurements and a theoretical investigation of the in-plane effective mass in the two-dimensional electron gas of nominally undoped AlGaN/GaN heterostructures with a different degree of quantum confinement. It is shown that in most cases the conduction band nonparabolicity effect is overestimated and the electron wave-function penetration into the barrier layer should be taken into account. The contribution of the wave-function hybridization is determined to play the dominant role. The band edge effective mass value is deduced to be (0.2±0.01)m0.

Interface structural defects and photoluminescence properties of epitaxial GaN and AlGaN/GaN layers grown on sapphire

Overall characterization of the GaN and AlGaN/GaN epitaxial layers by X-ray diffractometry and optical spectral analysis is carried out. The layers are grown by metalloorganic gas-phase epitaxy on (0001)-oriented single crystal sapphire wafers. The components of strains and the density of dislocations are determined. The effects of strains and dislocations on the photoluminescence intensity and spectra are studied. The results allow better understanding of the nature and mechanisms of the formation of defects in the epitaxial AlGaN/GaN heterostructures.

Electric Current and Noise in Long GaN Nanowires in the Space-Charge Limited Transport Regime

We studied electric current and noise in planar GaN nanowires (NWs). The results obtained at low voltages provide us with the estimates of the depletion effects in the NWs. For larger voltages, we observe the space-charge limited current (SCLC) effect. The onset of the effect clearly correlates with the NW width. For narrow NWs, the mature SCLC regime was achieved. This effect has great impact on fluctuation characteristics of studied NWs. At low voltages, we found that the normalized noise level increases with decreasing NW width. In the SCLC regime, a further increase in the normalized noise intensity (up to 104 times) was observed, as well as a change in the shape of the spectra with a tendency towards slope −3/2. We suggest that the features of the electric current and noise found in the NWs are of a general character and will have an impact on the development of NW-based devices.

Nature of low-energy optical emission in doped AlGaN∕GaN heterostructures

Photoluminescence (PL) in modulation-doped and nominally undoped AlxGa1−xN∕GaN heterostructures was studied and compared with PL spectra of GaN films grown on sapphire substrates. It is demonstrated that optical emission in the energy range of 3.3–3.46eV related to the two-dimensional electron gas radiative processes can be completely suppressed in modulation-doped AlxGa1−xN∕GaN heterostructures. Instead of this, an intense broad long-wavelength emission attributed to the recombination of donor-acceptor pairs in the lower energy range of 2.7–3.3eV is revealed. This spectral transformation is explained by the presence of deep-level defect-related acceptor centers in AlxGa1−xN∕GaN heterostructures introduced at the modulation doping of the AlxGa1−xN barrier layer.

Formation mechanism of contact resistance to III–N heterostructures with a high dislocation density

The temperature dependences of the contact resistance ρc(T) of ohmic Pd-Ti-Pd-Au contacts to n-GaN and n-AlN wide-gap semiconductors with a high dislocation density are studied. The dependences ρc(T) for both contacts contain portions of exponential decrease ρc(T) and very weak dependence ρc(T) at higher temperatures. Furthermore, a plateau portion ρc(T) is observed in the low-temperature region for the Au-Pd-Ti-Pd-n-GaN contact. This portion appears only after rapid thermal annealing (RTA). In principle, the appearance of the plateau portion can be associated with preliminary heavy doping of the near-contact region with a shallow donor impurity and with doping during contact fabrication as a result of RTA, if the contact-forming layer contains a material that is a shallow donor in III–N. The dependences obtained are not explained by existing charge-transport mechanisms. Probable mechanisms explaining the experimental dependences ρc(T) for ohmic contacts to n-GaN and n-AlN are proposed.

AlGaN/GaN heterostructures for hot electron and quantum effects

This paper reports on the transport and material characterization of AlGaN/GaN heterostructures. We compared the overheating temperatures in the transmission line model patterned devices during normal operation and analyze their dependence on the buffer thickness in order to optimize the thermal budget of the structures. It is demonstrated that noise spectra can be used to monitor the transport mechanisms and determine the activation energy of the traps. Small-dose gamma irradiation is used as an effective treatment for improving the structural properties. Based on cathodoluminescence and X-ray diffraction spectra shape, we explain the improvement as a result of the relaxation of elastic strains and structural-impurity ordering in the AlGaN barrier layer under irradiation. An irreversible improvement in mobility of the electrons in the channel, controlled by a proper dose of gamma radiation treatment, at considerably reduced self-heating is promising to achieve novel quantum transport regimes on the nanoscale.

Optical and structural study of deformation states in the GaN/AlN superlattices

We report on the effect of strain on the optical and structural properties of 5-, 10-, and 20-period GaN/AlN superlattices (SLs) deposited by plasma-assisted molecular beam epitaxy. The deformation state in SLs has been studied by high resolution transmission electron microscopy (HRTEM), X-ray diffraction, and micro-Raman, Fourier transform infrared (FTIR), and photoluminescence spectroscopy. HRTEM images showed that the structural quality of the SL layers is significantly improved and the interfaces become very sharp on the atomic level with an increase of the SL periods. A combined analysis through XRD, Raman, and FTIR reflectance spectroscopy found that with increasing number of SL periods, the strain in the GaN quantum wells (QWs) increases and the AlN barrier is relaxed. Based on the dependence of the frequency shift of the E2High and E1TO Raman and IR modes on the deformation in the layers, the values of the biaxial stress coefficients as well as the phonon deformation potentials of these modes in b...

Noise spectroscopy of AlGaN/GaN HEMT structures with long channels

In this paper we discuss the effect of dissipated power on the characteristic time of noise spectra transformation investigated in wide-bandgap AlGaN/GaN transistor structures with long channels. It is found that the characteristic time as a function of temperature demonstrates an exponential dependence with definite activation energy. Obtained results are explained based on the developed model of non-equilibrium fluctuations of the sample resistance. These fluctuations cause a local overheating due to local change of resistance. In the long channels these fluctuations may create overheating regions with two different self-heating temperatures, leading to different activation dependences, observed in the experiment.

Reliability and improved performance of AlGaN/GaN high electron mobility transistor structures

We present a systematic study of the impact of layer structure design on the channel temperature of AlGaN/GaN high electron mobility transistor (HEMT) structures. Device layer structures have been optimized to obtain minimum overheating temperature at high dissipated power in channel of HEMTs grown on different substrates. It is shown that temperature increase has opposite dependence on buffer thickness for sapphire and SiC substrates. Noise spectroscopy is also used to monitor the self-heating effect. Moreover, it is shown that the room temperature spectra can be used to determine the activation energy of the traps. An irreversible improvement in mobility is registered after irradiation of AlGaN/GaN heterostructures at a total dose 1x106 rad of 60Co gamma rays.