Conductivity Of GaN Research Articles

A conductivity-based selective etching for next generation GaN devices

Electrochemical etching having large selectivity based on the conductivity of n-type GaN was investigated to demonstrate the feasibility of novel optical and microelectromechanical system devices. The electrochemical etching exhibited two regimes with different etching characteristics, i.e., nanoporous and electropolishing, depending on the doping concentration and applied voltage. For photonic applications, GaN microdisks and distributed Bragg reflectors were fabricated where optical index contrast can be achieved by selective etching or nanoporous formation of GaN. Stimulated emission of GaN microdisk was observed under pulsed optical pumping. In addition, a GaN cantilever was formed and its resonance frequency was measured at ∼ 120 kHz.

Theory of the lattice thermal conductivity in bulk and films of GaN

We report on a systematic theoretical investigation of the lattice thermal conductivity of several GaN samples (bulk and films) over a wide range of temperature, by applying Callaway's relaxation-time theory in its full form and Srivastava's scheme for anharmonic three-phonon scattering processes. The role of the usually neglected three-phonon normal-drift term has been quantified. We have attempted to quantify the role of phonon scattering by various defects and imperfections, present in the film samples, in controlling the temperature dependence of thermal conductivity. We find that except for the purest sample, the phonon-impurity scattering plays a significant role in controlling the thermal conductivity of GaN not only around the thermal-conductivity peak region but also over a very large range of temperature. It can also be predicted from our numerical study, and with available experimental results, that the highest possible thermal conductivity of bulk GaN can only be realized when point impurities such as oxygen and silicon are in small concentration ($\ensuremath{\simeq}{10}^{16}\text{ }{\text{cm}}^{\ensuremath{-}3}$ or less) and other defects are either absent (from experimental study) or present in very small concentration (our numerical study). The highest value of the room-temperature thermal conductivity is achieved for samples grown by the high-temperature and high-pressure growth technique.

The level of local charge-neutrality and pinning of the Fermi level in irradiated nitrides wz-III-N (BN, AlN, GaN, InN)

On the basis of the density functional theory and general gradient approximation and the method of special points, the electronic spectra and the energy position of the local charge-neutrality level are calculated for the wurtzite BN, AlN, GaN, and InN compounds with the use of different heuristic models. It is shown that, with an increasing atomic mass of the cation in the wz-III-N compounds, the charge-neutrality level shifts from the position near the midgap in the BN and AlN, to the upper part of the band gap in GaN, and to the allowed energy region of the conduction band in InN. Such shifts define the semi-insulator properties of BN and AlN, the n-type conductivity of GaN, and the n+-type conductivity of InN on saturation of the wz-III-N compounds with intrinsic defects induced by high-energy irradiation.

The charge neutrality level and the fermi level pinning in A3N (BN, AlN, GaN, InN) nitrides

The electronic band structure and position of the charge neutrality level (CNL) in BN, AlN, GaN, and InN compounds with cubic and hexagonal lattices are calculated within the density functional theory (DFT-GGA). It is shown that the charge neutrality level is shifted from the middle of the BN and AlN forbidden band to the upper half of the GaN forbidden band and to the allowed energy region in the InN conduction band as the cation atomic weight increases. This determines semiinsulating properties of BN and AlN, n-type conductivity of GaN, and n +-type conductivity of InN upon saturation of these materials by intrinsic lattice defects due to hard radiation.

Periodic Mg distribution in GaN:δ-Mg and the effect of annealing on structural and optical properties

High p-type conductivity of GaN – commonly achieved via doping with Mg – is a challenge to be faced on the way to the realization of a GaN-based diluted magnetic semiconductor showing ferromagnetism at room temperature. In this work we present a study of GaN:Mg films grown in a metalorganic vapor phase epitaxy process where the Mg atoms are incorporated in a δ-doping fashion. The effect of a post-growth annealing step on the diffusivity of magnesium is studied by means of atomic force microscopy and photoluminescence measurements in the ultraviolet regime. A comparison of the presented results with transmission electron microscopy studies leads to the conclusion that a considerable amount of Mg is transported towards the surface where it is thermally removed during the annealing step.

Study of interface barrier of SiNx/GaN interface for nitrogen-polar GaN based high electron mobility transistors

The SiNx/GaN interface barrier height for N-polar GaN based metal-insulator-semiconductor high electron mobility transistors (MISHEMTs) was investigated. N-polar SiNx/GaN/AlGaN/GaN MISHEMT structures with different GaN cap thicknesses were grown by metal-organic chemical vapor deposition. The properties of the SiNx/GaN interface are of critical importance to device operation and modeling in these devices. An analytical expression for the pinch-off voltage of the HEMT was obtained, and capacitance-voltage (C-V) measurements with different Schottky metals were used to extract the barrier height. The Fermi level at the interface was found to be pinned at approximately 1 eV with respect to GaN conduction band edge, irrespective of the work function of the gate metal. Hall measurements of the two-dimensional electron gas density were found to corroborate the predicted interface barrier height. An approximate value for interface charge causing this pinning was calculated to be 4.5×1012 cm−2.

Effects of resonant LO phonon assisted excitation on the photoluminescence spectra of InGaN/GaN quantum wells

AbstractWe have observed oscillations in the low temperature (T < 50 K) photoluminescence excitation spectra of an InGaN/GaN single quantum well at photon energies above the GaN bandgap. We attribute the features in the spectra to excitation of electrons at multiples of the LO phonon energy above the GaN conduction band edge. The rapid cooling of these electrons to the GaN conduction band edge and their subsequent capture leads to a shift, in the photoluminescence spectrum and hence the oscillations in photoluminescence. We interpret the shift in the spectrum as being due to a modification of the occupation of the distribution of localization centres by electrons. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Cross-sectional scanning photoelectron microscopy and spectroscopy of wurtzite InN∕GaN heterojunction: Measurement of “intrinsic” band lineup

A method for studying heterojunction band lineups on the submicrometer scale is demonstrated by using synchrotron-radiation photoelectron microscopy and spectroscopy. In particular, an in situ sample cleavage technique is adopted here to reveal the cross-sectional, nonpolar a-plane face of InN∕GaN heterojunction grown on Si(111) along the polar −c axis with fully relaxed lattice structure, eliminating the polarization effects associated with the interface charge/dipole normal to the cleaved surface. The “intrinsic” valence band offset at the cleaved InN∕GaN heterojunction has been determined to be 0.78eV. Additionally, using known material parameters, the values of InN∕GaN conduction band offset and InN electron affinity are also estimated.

Size-quantized oscillations of the electron mobility limited by the optical and confined acoustic phonons in the nanoscale heterostructures

The authors theoretically investigated the electron mobility in the nanometer thickness AlN∕GaN∕AlN heterostructures limited by the polar optical and confined acoustic phonons. The proposed model accurately takes into account dispersion of the optical and acoustic phonons in such heterostructures as well as inelasticity of the electron scattering on both optical and acoustic phonons. It has been shown that the intersubband electronic transitions play an important role in limiting the electron mobility when the energy separation between one of the size-quantized excited electron subbands and the Fermi energy becomes comparable to the optical or confined acoustic phonon energy. The latter results in the nonmonotonic oscillatory dependence of the electron mobility on the thickness of the GaN conduction channel layer. The predicted effect is observable at room temperature and over a wide range of carrier densities. The described mechanism can be used for fine tuning the confined electron and phonon states in the nanoscale heterostructures made of different material systems in order to achieve performance enhancement of the nanoscale electronic devices.

Effect of isotope on lattice thermal conductivity of lateral epitaxial overgrown GaN

A theoretical model of the lattice thermal conductivity of GaN is presented on the dependence of isotope abundance and impurity. By using a modified Callaway's model, the thermal conductivity is evaluated and compared with experimental data. The effect of isotope, dislocation and impurity are all taken into account. It is found that there are critical values of impurity concentration and dislocation density below which the isotope effect on conductivity has to be considered and beyond which impurity and dislocation have significant impact. For LEO GaN with a lower dislocation density than normal GaN, the effect of the isotope becomes significant rather than negligible especially in the mask area which has a low dislocation density.

Fabrication and Electrical Characterization of Be-Doped Al0.42Ga0.58N MSMs

One of the greatest challenges facing GaN devices has been efficient p-type doping. Beryllium is a promising dopant for obtaining p-type conductivity in GaN. An investigation has been made into ionizing Be acceptors in AlGaN devices that has varying levels of Be implantation. The AlGaN samples had MSM structures placed on the doped surface in order to measure electrical characteristics. I-V characteristics were measured at room temperature without optical pumping. Future measurements with optical pumping through the use of a YAG IR laser are planned.

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

Abstract 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 ( 10 1 5 ) 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 Schrodinger–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.

Effects of plasma treatment on the Ohmic characteristics of Ti∕Al∕Ti∕Au contacts to n-AlGaN

The effects of surface treatment using Cl2∕BCl3 and Ar inductive coupled plasmas on the Ohmic characteristics of Ti∕Al∕Ti∕Au contacts to n-type AlxGa1−xN (x=0–0.5) were investigated. Plasma treatment significantly increased the surface conductivity of GaN and Al0.1Ga0.9N, leading to improved Ohmic behaviors of the contacts. However, it reduced the surface doping level in AlxGa1−xN (x⩾0.3) and degraded the contact properties. Following a 900–1000°C anneal, the Ti∕Al∕Ti∕Au contacts to AlxGa1−xN (x=0–0.3) became truly Ohmic, with specific contact resistances of (3–7)×10−5Ωcm2, whereas the contact to Al0.5Ga0.5N remained rectifying even without the plasma treatment. X-ray photoelectron spectroscopy measurements confirmed that the Fermi level moved toward the conduction band in GaN after the plasma treatment, but it was pinned by plasma-induced deep-level states in Al0.5Ga0.5N. This study emphasizes the need to mitigate plasma damage introduced during the mesa etch step for AlGaN-based deep-UV emitters and detectors.

Thermal conductivity of GaN

Abstract Thermal conductivity (TC) data on wurtzite GaN samples, in literature, covering a wide temperature range (4.2–400 K), are analyzed employing two models; one due to Holland and another a modified Callaway model. Scattering of phonons by sample boundaries, impurities, dislocations and other phonons via both normal and umklapp processes is considered. A good representation of the temperature dependence, in all the samples considered, is obtained with a single set of phonon–phonon scattering parameters. The parameters are found to be reasonable and consistent with values in other systems. The parameters characterizing the scattering by various imperfections are also determined. The analyses presented bring out the relative importance of the phonon scattering mechanisms and the characteristic temperature dependencies operative in GaN. A need for inclusion of more realistic phonon dispersion and relaxation times is pointed out.

Impact of substrate temperature on the incorporation of carbon-related defects and mechanism for semi-insulating behavior in GaN grown by molecular beam epitaxy

The electrical conductivity and deep level spectrum of GaN grown by molecular beam epitaxy and codoped with carbon and silicon were investigated for substrate temperatures Ts of 650 and 720°C as a function relative carbon and silicon doping levels. With sufficiently high carbon doping, semi-insulating behavior was observed for films grown at both temperatures, and growth at Ts=720°C enhanced the carbon compensation ratio. Similar carbon-related band gap states were observed via deep level optical spectroscopy for films grown at both substrate temperatures. Due to the semi-insulating nature of the films, a lighted capacitance-voltage technique was required to determine individual deep level concentrations. Carbon-related band gap states underwent substantial redistribution between deep level and shallow acceptor configurations with change in Ts. In light of a Ts dependence for the preferential site of carbon incorporation, a model of semi-insulating behavior in terms of carbon impurity state incorporation mediated by substrate temperature is proposed.

Temperature and Dislocation Density Effects on the Thermal Conductivity of Bulk Gallium Nitride

The performance of III-Nitride high power, high frequency transistors and laser diodes is intimately connected with the ability to dissipate heat from the junction to the substrate. The thermal conductivity was characterized by the three omega method for undoped and doped gallium nitride bulk substrates grown by HVPE from room temperature to 450 K. The thickness of the samples varied from thin film epilayers on sapphire to 2 millimeter thick free standing samples. Dislocation density of the substrates was measured by imaging cathodoluminescence, SIMS was used to measure impurity levels of oxygen, hydrogen, silicon, and iron, while carrier concentrations and resistivity were determined from electrical measurements and EPR. A semi-insulating, 2 mm thick iron doped sample had the highest thermal conductivity of 230W/K-m at room temperature. Undoped samples had comparable, but lower thermal conductivities throughout the temperature range from 300-450 K. By comparing these results with previously reported experimental results including those on MOCVD grown GaN free of grain boundaries, we establish an empirical relationship in a compact formula that relates the thermal conductivity of GaN and the dislocation density with three different regimes of low, intermediate, and high dislocation densities. In the high dislocation regime, the thermal conductivity improves significantly with reduction of dislocation densities. As material quality continues to improve it remains to be seen if in the low dislocation density regime, thermal conductivities will approach 300 W/K-m or plateau out near 250 W/K-m. As point defects start to limit the thermal conductivity when dislocation density becomes very low, gallium vacancies are expected to play an increasing role. Iron is postulated to substitute on the gallium site. The indication from this study is that iron doping at concentration of 1018 cm−3 is not limiting the thermal conductivity in the 300-450 K range.

17aB11 分子動力学法による格子欠陥を考慮したGaNの熱伝導率解析(半導体エピ(2),第35回結晶成長国内会議)

17aB11 分子動力学法による格子欠陥を考慮したGaNの熱伝導率解析(半導体エピ(2),第35回結晶成長国内会議)

Investigation of thermal conductivity of GaN by molecular dynamics

Thermal conductivity of GaN was investigated by molecular dynamics simulation. We used Stillinger–Weber potentials, and Green-Kubo's formula was employed to calculate thermal conductivity. The results showed that the thermal conductivity of GaN at 300 K was in the range of 310 < λ < 380 W / mK , which is higher than the reported experimental values. We also investigated the temperature dependence of thermal conductivity in the temperature range of 300–1200 K.

Electrical and surface composition properties of phosphorus implantation in Mg-doped GaN

Changes in electrical as well as surface composition such as chemical and electronic properties of Mg-doped p-type GaN by phosphorus implanting are systematically investigated using Hall effect and X-ray photoelectron spectroscopy (XPS) measurements. It is shown that p-type conductivity of Mg-doped GaN can be improved by implanting P atoms after a proper post-implantation annealing treatment, probably due to the reduction of self-compensation by P atoms substitution on N vacancy sites. XPS analysis is further found that the decrease of surface oxides and the shift of the surface Fermi level toward the valence band edge through P atoms introduced. These experimental results indicate that the P implantation is an effective method to improve p-type conductivity of Mg-doped GaN and reducing the surface barrier height, which can lead to a lower metal contact resistivity to p-type GaN.

Optoelectronic devices on bulk GaN

The homoeptaxial fabrication of GaN-based devices has advantages against heteroepitaxial realization on substrates such as sapphire or SiC, since heteroepitaxy implies a lot of problems like lattice mismatch, different thermal expansion coefficients, and needs an extensive optimization of the growth at the heterointerface. In this paper we will discuss GaN-based light-emitting devices grown by homoepitaxy in comparison to devices grown on sapphire. A special emphasis is laid on the pretreatment of the GaN substrate and the device characteristics on different substrates. In detail will be discussed the advantages of the higher thermal conductivity of GaN and how this effects the device performance.