The influence of sample orientation and polarity on the blue emission from As-doped GaN layers grown by plasma-assisted molecular beam epitaxy (PA-MBE) was investigated. Arsenic doped GaN layers were grown under identical PA-MBE conditions on several types of substrates including c-plane (0001) sapphire and polar and non-polar GaN templates grown by metal-organic vapour phase epitaxy (MOVPE). Non-polar GaN MOVPE templates were grown on aplane (11–20) sapphire and LiAlO2 (100). The orientation and polarity have a strong influence on the morphology and the optical properties of As-doped GaN layers. Strong blue emission from As-doped GaN was observed only in the case of (000–1) oriented N-polarity layers.

The investigated InGaN ternary films were grown by MOMBE, with the In fraction varying from 6% to 100%. Using XRD and TEM, we determine the crystalline state, the nature of defects inside the layers and their distribution as related to the In concentration. The band gap of the alloys was studied through PL measurements, it is below 1 eV for InN.

We report on the novel normally-off AlGaN/p-type GaN heterojunction field effect transistors (HFETs). We grew the AlGaN/p-GaN heterostructure on p-type Si (111) substrate using a metalorganic chemical vapor deposition (MOCVD). A homogeneous buffer layer was first formed on a Si (111) substrate at 1123 K. After that, AlGaN (30 nm)/high resistive p-type GaN (500 nm) heterostructure was also grown at 1303 K without cracking. We fabricated an HFET using AlGaN/p-type GaN on Si substrate. The ohmic electrode material was Al/Ti/Au and the gate electrode was Pt/Au. The distance between the source and the drain was 0.01 mm. The gate length and width were 2000 nm and 0.15 mm, respectively. As a result, the HFET was operated at the condition of the positive gate bias. The pinch-off voltage was 0 V. A normally-off operation was thus confirmed for the first time. The breakdown voltage of FET was over 250 V. We also confirmed that the normally-off HFET was operated at 573 K for over 150 h.

The local structure around Er and Eu atoms introduced into GaN epilayers was studied by means of Extended X-ray Absorption Fine Structure above the appropriate rare-earth X-ray absorption edge. The samples were doped in situ during growth by Molecular Beam Epitaxy. The formation of ErN clusters was found in samples with high average Er concentrations of 32±6% and 12.4±0.8%, estimated by Wavelength Dispersive X-ray analysis. When the average Er concentration is decreased to 6.0±0.2%, 1.6±0.2% and 0.17±0.02%, Er is found in localised clusters of ErGaN phase with high local Er content. Similar behaviour is observed for Eu-doped samples. For an average Eu concentration of 30.5±0.5% clusters of pure EuN occur. Decreasing the Eu concentration to 10.4±0.5% leads to EuGaN clusters with high local Eu content. However, for a sample with an Eu concentration of 14.2±0.5% clustering of Eu was not observed.

Deep traps in semi-insulating (SI) or high-resistivity C-doped GaN grown by metal-organic chemical-phase deposition or molecular-beam epitaxy have been studied by thermally stimulated current (TSC) spectroscopy. Incorporation of carbon in GaN introduces CN acceptors, resulting in compensation and formation of SI-GaN; however, as [C] increases in the GaN samples, both resistivity and activation energy of the dark current decrease. In the GaN samples with low [C], we find at least six TSC traps: B (0.61 eV), Bx (0.50 eV), C1 (0.44 eV), C (0.32 eV), D (0.23 eV), and E (0.16 eV), all of which are very similar to electron traps typically found in n-type GaN by deep level transient spectroscopy (DLTS). However, in the GaN sample with the highest [C], both traps E and B are suppressed, and instead, trap Bx appears. Based on DLTS studies of electron-irradiated and plasma-etched GaN samples, we believe that traps E, D and C are related to VN, and that trap B is probably related to VGa, in the form of complexes such as VGa-ON. As [C] increases, CGa donors become more favorable, and the transition of trap B to trap Bx may suggest that CGa related complexes are forming. In comparison with lightly C-doped GaN, heavily C-doped GaN sample exhibits very strong PPC at 83 K. We show that the PPC in both cases can be simply explained by the thermal emission of carriers from shallower traps.

Homogeneous GaNAs alloy films containing 3 to 4 percent of arsenic anion fraction were synthesized by metalorganic chemical vapor deposition (MOCVD). Post-growth annealing resulted in the formation of GaAs precipitates of several nanometers in size embedded in the nitrogen-rich GaN(As) matrix, while as-grown ternary films exhibited no evidence of phase segregation. Band gap reduction due to alloying was observed by the optical transmission measurements, leading to an estimate for the bowing parameter of 25 eV in the films with the arsenic content of 3.5 %.

We have investigated high-power characteristics of GaN/InGaN double heterojunction bipolar transistors on SiC substrates grown by metalorganic vapor phase epitaxy. The p-InGaN extrinsic base layers were regrown to improve ohmic characteristics of the base. Base-collector diodes showed low leakage current at their reverse bias voltages due to a wide bandgap of a GaN collector, resulting in a high-voltage transistor operation. A 90 μm × 50 μm device operated up to a collector-emitter voltage of 28 V and a collector current of 0.37 A in its common-emitter current-voltage characteristics at room temperature, which corresponds to a DC power of 10.4 W. A collector current density and a power density are as high as 8.2 kA/cm2 and 230 kW/cm2, respectively. These results show that nitride HBTs are promising for high-power electronic devices.

We report a comparative study of the exciton emission in free standing HVPE layer for all polarization configurations. A noticeable difference between the emission spectra polarized perpendicular and parallel to the c-axis of the crystal is observed. The spectra for E┴c and EIIc are found to be dominated by the emissions of the donor-bound exciton and exciton-polariton both arising from the A and B valence band, respectively, which clearly reveals the optical selection rules in wurtzite GaN. The temperature evolution of the emission spectra is also examined and the thermal redistribution of the excitons at different polarization is discussed.

Reactive MBE growth was used to synthesize ferromagnetic Cr-doped GaN and AlN thin films with Curie temperatures above 900K. 2% Cr-doped GaN and 7% Cr-doped AlN were found to have a saturation magnetization moment of 0.42 and 0.6 μB/Cr atom, indicating that ∼14% and ∼20% of the Cr, respectively, are magnetically active. Structural characterization using X-ray diffraction (XRD) and transmission electron microscopy (TEM) did not find evidence of a ferromagnetic secondary phase. Electrical characterization indicate that the resistivity of the Cr-doped GaN films depends exponentially on temperature as R=Roexp[(To/T)1/2], characteristic of variable range hopping. In contrast, Cr-doped AlN films are highly resistive. Local spin density functional calculations predict that Cr forms a deep level defect in both systems and the t2 level falls approximately at midgap. Our theoretical and experimental results indicate that ferromagnetism in Cr-doped GaN and AlN arises as a result of the double exchange mechanism within the partially filled Cr t2 band.

The performance of AlGaN/GaN High Electron Mobility (HEMT) transistors is directly related to the electrical characteristics of the two-dimensional electron gas formed at the interface thanks to the piezoelectric field. Modification of the Al content or thickness of the AlGaN layer can within a certain limit modify the carrier density and mobility in the 2DEG. However, further reduction of the sheet resistance requires strain engineering of the heterostructure. An effective way to reduce the sheet resistance, as well as to lower the threading dislocation (TD) density, is to perform strain engineering through the use of low temperature AlN interlayers inserted in the GaN buffer layer. From correlation of AFM, TEM and HRXRD mapping of the HEMT layers, the strain modification, as well as the mechanism reducing the TD density, can be explained by the highly defected nature of the AlN interlayer grown at low temperature, as well as its very small thickness. The LT AlN acts as a second nucleation layer for the GaN grown on top. Contrarily, when the AlN interlayer is grown at 1050°C, its high crystalline quality and the possibility to grow pseudomorphic and abrupt interfaces, allows its use at the AlGaN/GaN interface. Optimal combination of the AlGaN and AlN layer thickness leads to record values of the mobility at room temperature of 2050 cm2/Vs, for heterostructures grown on sapphire, which is approaching state-of-the-art for HEMT grown on SiC.

Ab initio total energy calculations, based on the full-potential augmented plane wave method within the local density approximation, are used to study the influence of alloying on the structural and electronic properties of cubic InxGa1-xN and InxAl1-xN ordered alloys in the chalcopyrite and luzonite structures.

We have investigated the lattice parameters and band gap energies. The lattice parameters, exhibit an upward bowing of -0.05 Å and -0.15 Å, respectively for the InxGa1-xN and InxAl1-xN alloys. The composition dependence of the band gap shows a bowing parameter of 1.36 eV for InxGa1-xN. The band gap InxAl1-xN alloys exhibits a bowing parameter of 3.19 eV, and a direct to indirect band gap transition at x (Al) = 0.83. The large bowing effect in InxAl1-xN alloys has been discussed in terms of a composition dependent bowing parameter.

Growth of AlN bulk single crystals by sublimation of AlN powder was carried out in a resistively heated reactor at temperatures up to 2300°C. A variety of crucible materials, such as BN, W, Ta, Re, ZrO2, TaN, and TaC, were evaluated. Our studies have shown that the morphology of crystals grown by spontaneous nucleation strongly depends on the growth temperature and contamination in the reactor. Crucible selection had a profound effect on contamination in the crystal growth environment, thus influencing nucleation, coalescence, and crystal morphology. Spontaneously grown single crystals up to 15 mm in size were characterized by x-ray diffraction (XRD), x-ray topography (XRT), glow discharge mass spectrometry (GDMS), and secondary ion mass spectrometry (SIMS). Average dislocation densities were on the order of 103 cm−3, with extended areas virtually dislocation-free, while high-resolution XRD showed rocking curves as narrow as 7 arcsec.

AlGaN/GaN based high power, high frequency high electron mobility transistors (HEMTs) have been in development for over a decade. Although much progress has been made, AlGaN/GaN HEMT technology has yet to be commercialized. The choice of silicon as the substrate for the growth of GaN-based epi layers will enable commercialization of AlGaN/GaN based HEMTs, because of its maturity, scalability, reproducibility and economy. One of the epitaxial issues pertaining to the growth of AlGaN/GaN HEMTs on Si is the understanding of parasitic losses that can adversely impact the RF device performance. The effect of the III-N MOCVD process on the resistivity of the Si substrate, and correlations between the Si substrate resistivity and AlGaN/GaN HEMT RF characteristics are presented. Optimization of the MOCVD growth process led to a reduction in parasitic doping of the Si substrate. This resulted in the following improvements: (a) small signal gain increased from 17 to 21dB, (b) the cut-off frequency increased from 7 to 11GHz and (c) the maximum frequency of oscillation improved from 12 to 20GHz. This optimized process will enhance performance of AlGaN/GaN HEMTs at higher frequencies.

One of the principle problems of high power electronic devices is the extraction of heat from the active region of the device. The thermal conductivity of the substrate is a crucial parameter affecting the thermal dissipation capability of a device. In this study we investigated the thermal conductivity at room temperature of five GaN samples grown on silicon, employing the 3 omega thermal conductivity method. The thickness of the GaN layers varied from 0 to 700 nm. The effective thermal conductivity for the GaN layers was found to range from 130 to 140 W/m·K which is comparable to thermal conductivity of the silicon substrate. This result indicates that the heat transfer of GaN on silicon is as good as either GaN or silicon alone, and that no substantial thermal degradation attributable to the GaN/Silicon interface is observed.

The electrical activity of iron in Fe- doped, and in Si and Mg co-doped GaN layers grown on sapphire substrates by metal organic vapor phase epitaxy was studied as shown by temperature dependent Hall Effect (TDH) measurements. In all samples iron doping generates an acceptor defect, which compensates donors in n-type GaN. Furthermore, iron doping causes strong potential inhomogeneities, which decrease the Hall mobility in the layers. To verify, if iron creates only hole traps, defects in n-type Si:Fe and Fe doped samples were investigated. The well known dominant electron traps in n-type GaN at 520 – 550 meV and 480 meV were found by deep level transient spectroscopy and thermal admittance spectroscopy, respectively. A high Fe-doped GaN layer shows a low p-type conductivity dominated by the iron acceptor. An activation energy of EV+ 460 meV was determined by TDH indicating, that the iron acceptor correlates with this defect level.

Pseudomorphically strained epitaxial films of the ternary alloy GaNyAs1-y have been grown on GaAs(100) with y ranging from 0 to 0.05. The optical phonon Raman spectrum of the alloy displays a two-mode behavior. The GaAs-like first order modes are represented at y = 0.05 by the strong longitudinal optic (LO1) mode at 288.5 cm-1 and the weaker transverse optic (TO1) mode at 268.3 cm-1, while the GaN-like LO2 mode is observed at 474.8 cm-1. Two very broad disorder-induced acoustic bands are evident at 80 and 170 cm-1 due to atomic disorder within the crystalline network. Raman studies show that as the nitrogen concentration increases, the GaAs-like LO1 band shifts linearly towards lower wavenumber while the GaN-like LO2 phonon band displays a sub-linear increase in wavenumber. Raman results for the unstrained quaternary alloy In0.06Ga0.94N0.02As0.98 are compared with those of GaN0.02As0.98.

The growth of high-quality GaN by plasma assisted molecular beam epitaxy (MBE) is challenging, in part due to the constraint of heteroepitaxy since GaN substrates are not yet commercially available and isotropic nature of growth. Despite the large lattice and thermal mismatch between sapphire and GaN, the former is still the most commonly used substrate for the GaN-based optical devices at present. In this paper, we demonstrate a re-growth technique to obtain an improved quality GaN by MBE on GaN template on sapphire where the grossly defective regions have been removed. This GaN template is formed by MBE growth of GaN followed by wet chemical etching to selectively remove the defective region. Improved quality GaN was re-grown on such a template under Ga rich conditions to a thickness of about 1 micron. After re-growth, the surface of GaN is atomically smooth with spiral features in the short range. The low temperature PL of the re-grown GaN is superior to those of MBE GaN films directly on sapphire. Atomic force microscopy (AFM) images reveal a two-dimensional re-growth initiating in regions free of extended defects. The results show that the selectively etched GaN on sapphire can be used as a good template to improve the quality of GaN.

We study the degradation behaviour of GaN gain guided laser diodes (LDs) on SiC substrates with cleaved facets and reflective coatings on none, one, or both facets. This allows us to demonstrate that in addition to volume effects there is a contribution of the laser facets to laser degradation. We observe that for the uncoated LDs the threshold current density is increasing considerably faster compared to LDs with mirror coatings. Degradation is observed during operation but not during storage at ambient conditions and thus expected to be photon or current induced. Operation of the uncoated laser in a nitrogen atmosphere reduces the degradation rate with respect to operation in air.

In this study we propose a novel method to increase hole concentration introducing isovalent substitutional dopant into a p-type nitride semiconductor. Acceptor dopant in nitride semiconductors makes deep acceptor level (>100 meV) and generates few hole carriers into the valence band because of large electron affinity of N.In contrast to this, substitution of isovalent group-V atoms (P, As and Sb) that has smaller affinity than N makes a localized group-V band upward the valence band maximum (VBM). When both acceptor and isovalent group-V atoms are incorporated into nitride semiconductor, holes can be drastically activated by isovalent atoms, and can easily move in the group-V band. We have also investigated this material, Mg-doped AlN:V (V=P, As or Sb), using first-principles pseudopotential method. As a calculation result, substitiution of P and As makes localized group-V band upward the VBM of AlN, and moreover this can be adjusted the VBM of GaN. The Mg incorporation into AlN:V as an acceptor dopant drastically decreases the Fermi level (ΔEF=-0.10 eV), that is, hole concentration can be drastically raised by the group-V band. Consequently novel p-type material with isovalent dopant can be a candidate to efficiently inject hole current into the VBM of GaN.

We measured the absolute value of the surface band bending in GaN layers grown by molecular beam epitaxy with a charge sensitive surface microprobe. Surface potential measurements showed an upward band bending from 0.7 to 1.4 eV in undoped and Si-doped GaN. The samples stored in dark for one week showed an increase in band bending by up to 0.1 eV. The effect of ultraviolet (UV) exposure (with a lamp or a pulsed nitrogen laser) on band bending was also studied. Typically, the surface barrier decreased by about 0.2 – 0.5 eV under UV light. The barrier was restored very slowly (by a logarithmic law) in the dark at room temperature. These and other similar phenomena are tentatively attributed to thermionic transfer of free electrons from the bulk to the surface states. Photo-induced desorption of oxygen may also play a role in the observed effects.