Showing papers in "Physica Status Solidi (a) in 2002"

High Power LEDs - Technology Status and Market Applications

Abstract: High power light emitting diodes (LEDs) continue to increase in output flux with the best III-nitride based devices today emitting over 150 lm of white, cyan, or green light. The key design features of such products will be covered with special emphasis on power packaging, flip-chip device design, and phosphor coating technology. The high-flux performance of these devices is enabling many new applications for LEDs. Two of the most interesting of these applications are LCD display backlighting and vehicle forward lighting. The advantages of LEDs over competing lighting technologies will be covered in detail.

GaN-Based Devices on Si

Abstract: Nowadays, GaN-based devices are usually grown on sapphire or silicon-carbide substrates. These are either insulating or very expensive and not available in large diameter. A well-conducting low-cost alternative is silicon also enabling the integration of optoelectronics or high-power electronics with Si-based electronics. The main problem limiting a fast progress of GaN growth on silicon is the thermal mismatch of GaN and Si leading to cracks even below device-relevant layer thicknesses. In the last few years, since the first demonstration of a molecular beam epitaxy grown GaN-based light emitting diode on Si in 1998 the activities in research of GaN on Si increased dramatically. Meanwhile, several concepts to lower stress, avoid cracks, and improve the material quality exist. Meanwhile the material quality has improved significantly and is about as good as on sapphire so that it is only a question of time until GaN-based devices on Si come into market. This article gives a review on the latest developments in group-III nitride growth on Si by metal organic vapor phase epitaxy.

The strength of concentrated Mg-Zn solid solutions

Abstract: Solid solution effects on the hardness and flow stress have been studied for zinc contents between 0.2 and 2.4 at% (0.5 and 6.9 wt%) in Mg. The alloys were grain refined with 0.6 wt% zirconium to ensure a similar grain size at all compositions. The hardness increases with the zinc content as Hv(10) (kg mm(-2)) = 9 Zn (at%) + 33. At low solute concentrations the (0.2%) proof strength does not change significantly with concentration. At concentrations above 0.7 at%, within the supersaturated solid solution region, the rate of solid solution hardening is high, following a c(2) rule, where c is the atom fraction of Zn. It is suggested that short-range order may account for most of the observed strengthening in concentrated Mg-Zn alloys.

Photoluminescence and Raman spectra of ZnO thin films by Charged Liquid Cluster Beam Technique

Abstract: Zinc oxide (ZnO) thin films have been deposited on glass substrates from isopropyl solutions of zinc acetate by using the Charged Liquid Cluster Beam (CLCB) technique. The structure of the films is determined by the temperature of the substrate. Raman scattering spectra of the films obtained by non-resonant excitation are similar to those of bulk polycrystalline ZnO. The photoluminescence (PL) spectra of the films exhibit at 10 K four broad emission bands in the visible and near infrared spectral range. The effect of post-deposition annealing on the red emission band is also reported.

Structure and Magnetic Properties of La3+-Substituted Strontium Hexaferrite Particles Prepared by Sol–Gel Method

Abstract: Influences of the substituted amount of La 3 + on structure and magnetic properties of Sr 1 - x La x Fe 1 2 O 1 9 compounds prepared by the sol-gel method have systematically been investigated by XRD, VSM, and TEM. Optimum magnetic properties of σ s = 74.1 Am 2 /kg and H c J = 498.4 kA/m are achieved with material of the composition Sr 0 . 8 5 La 0 . 1 5 Fe 1 1 . 8 5 O 1 9 heated at 850 °C in air. It is found that the suitable amount of La 3 + substitution may remarkably increase the saturation magnetization σ s and the intrinsic coercivity H c J . The Curie temperatures of Sr 1 - x La x Fe 1 2 O 1 9 decrease linearly with increasing La 3 + substitution.

Effect of Vacuum Annealing on the Properties of Transparent Conductive AZO Thin Films Prepared by DC Magnetron Sputtering

Abstract: Using highly conductive Al doped ZnO (AZO) ceramic target, (001)-oriented transparent conductive AZO (Al doped ZnO) thin films are prepared by dc planar magnetron sputtering deposition on glass sheet substrates. The structural, electrical, and optical properties of the films (deposited at room temperature and annealed at different temperatures in vacuum) are characterized with various techniques. The experimental results show that electrical resistivity of the as-grown films (not intentionally heated) can be decreased to 10 -4 Ω cm level with post-deposition annealing at 400 °C for 2 h in vacuum pressure of 10 -5 Torr. AZO thin films deposited at room temperature (RT) and subsequently annealed at 450 °C for 2 h in vacuum have nano-crystalline structure with even crystal size distribution. With the increase of annealing temperature, the (002) preferred orientation of AZO thin film grows, the carrier concentration of AZO thin film increases, and the absorption edge in the transmission spectrum shifts toward the shorter wavelength side (blue shift). The oxygen vacancies have played an important role in the improvement of electrical conductivity of these AZO thin films with the vacuum annealing process.

One-dimensional Modelling of Thermoelectric Cooling

Abstract: Applying a one-dimensional formulation, the authors give a short overview on thermoelectric effects. Starting from the first principles of thermoelectricity, differential equations governing the coupling of thermal and electric transports have been analytically and numerically solved by the software tool MATHEMATICA. For (Bi 0.5 Sb 0.5 ) 2 Te 3 as an active material, the temperature profile T(x) in a single Peltier element has been calculated for constant thermoelectric material properties as well as for temperature dependent parameters. It is shown that temperature dependence effects can be sufficiently approximated by constant volume average values as far as the electric current remains near or below the optimum value for maximum temperature difference over the element.

High‐Power III‐Nitride Emitters for Solid‐State Lighting

Abstract: High-power, large-area InGaN/GaN quantum-well heterostructure light-emitting diodes based on an inverted, or flip-chip, configuration are described. These devices are mounted in specially designed high-power (1-5 W) packages and exhibit high extraction efficiency and low operating voltage. In the blue wavelength regime, output powers greater than 250 mW (1 x 1 mm 2 device) and 1 W (2 x 2 mm 2 device) are delivered at standard operating current densities (50 A/cm 2 ), corresponding to wall-plug efficiencies of 22%-23%. Employing phosphors for the generation of white light, these same devices achieve luminous efficiencies greater than 30 lm/W.

Preparation and study of structural and optical properties of chemical bath deposited copper indium diselenide thin films

Abstract: Copper indium diselenide thin films of different thicknesses were prepared by the chemical bath deposition technique onto well-cleaned glass substrates at room temperature. The thickness of the deposited films has been determined by gravimetry. The structural characterization was carried out by X-ray diffraction and scanning electron microscopic studies. These studies confirm the polycrystalline nature of the films with chalcopyrite structure. The structural parameters such as lattice constants, axial ratio, tetragonal distortion, crystallite size, dislocation density and number of crystallites per unit area have been evaluated. The composition of the various constituents in CuInSe 2 films has been determined by energy dispersive X-ray analysis. The optical properties have been studied in detail in the wavelength range 4000-14500 A and the optical band gap has been found to be direct and allowed.

UV Light‐Emitting Diode Fabricated on Hetero‐ELO‐Grown Al0.22Ga0.78N with Low Dislocation Density

Abstract: Using a hetero-epitaxial lateral overgrowth (ELO) technology with low-temperature deposited AlN interlayer, high-quality crack-free Al 0.22 Ga 0.78 N with low dislocation density is realized. The dislocation density in the Al 0.22 Ga 0.78 N was reduced to as low as the order of 10 7 cm -2 over the whole wafer. Applying the high-quality AlGaN layer to a UV light-emitting diode (LED), high output power of more than 0.1 mW at a forward current of 50 mA has been demonstrated with the emission peak wavelengths from 323 to 352 nm. The highest output power of 0.6 mW is obtained for the 352 nm LED with GaN/AlGaN multiple quantum well active layer. The emission efficiency was dependent on the wavelength, and the 323 nm LED has the lowest output power of 0.18 mW at 50 mA bias. One of the factors determining the external quantum efficiency is thought to be an inferior hole spread in highly resistive p-type layers.

Reduction of Internal Loss and Threshold Current in a Laser Diode with a Ridge by Selective Re-Growth (RiS-LD)

Abstract: The origin of the internal loss in ridge-type laser diodes (LDs) fabricated using selective re-growth is investigated through a systematic device characterization and additional optical measurements. We found that the internal loss of this LD is mainly caused by the absorptive'layers at the re-growth boundary and Mg-doped GaN layer. The internal loss can be significantly reduced through a re-design of the LD structure to avoid these absorptive regions by shifting the perpendicular optical field to the n-cladding side. The re-designed LDs had a very low threshold current of 10 mA and superior gain characteristics. These results indicate, that the InGaN-quantum-well (QW) active layer has a large differential gain and fewer non-radiative defects. The fabrication method of this LD, i.e. epitaxial growth on low-dislocation-density GaN substrates combined with a process without dry-etching, is responsible for the high quality of the QWs.

Physical Characteristics of Stabilized Silver Nanoparticles Formed Using a New Thermal-Decomposition Method

Abstract: We decomposed a silver-fatty acid complex at 250 °C in a nitrogen atmosphere to make stable, mono-disperse silver nanoparticles The size distribution was measured using high-resolution transmission electron microscopy (HRTEM); the mean particle diameter was 47 nm and the standard deviation was 06 nm This size distribution was verified by the measured surface-plasmon absorption spectrum Their X-ray diffraction pattern showed that they were metallic silver Then we examined their internal structure using HRTEM The particles cross-sectional shapes were usually circular except some were hexagons or pentagons According to the HRTEM image, most particles were multiple twins; 18% were simple twins and 2% were single crystals Use of thermogravimetry and differential thermal analysis showed that their surface was coated by fatty acid This coating stabilized the nanoparticles thus preventing their aggregation even after repeatedly dissolving them in solvent and evaporating However, the coating was easily removed by baking at 210 °C in one atmosphere of air

Microstructure Evolution in the Cemented Carbides WC–Co I. Effect of the C/W Ratio on the Morphology and Defects of the WC Grains

Abstract: The morphology and the interface features of WC grains are examined by SEM and TEM in cemented carbides WC-8 wt% Co. Three compositions - the mixture {WC + Co binder) and the mixtures with graphite or η - are studied after treatments at 1200 and 1450 °C. The effect of the C content is observed both on the WC crystal features and on the microstructure evolution. In all the alloys, large faceted grains that are defect free single crystals are formed at 1450 °C. On the C rich side, the increase in number and size of the large grains is more regular, the WC grains are more faceted and grow already during heating. On the W rich side, the WC grains have slightly rounded corners and interface steps. The C content influence on the interface energy anisotropy and the growth process of the WC grains is discussed from the microstructure results.

Improved electrical properties for metalorganic vapour phase epitaxial InN films

Abstract: A MOVPE InN film with a carrier concentration in the order of 10 18 cm -3 is successfully grown for the first time by using the atmospheric-pressure growth. Employing a reduced gas speed due to the atmospheric pressure and a reduced dilution N 2 gas flow rate, a carrier concentration of 5.8 × 10 18 cm -3 has been attained with a high electron mobility. The optimum growth temperature for the atmospheric pressure as well as reduced-pressure MOVPE growth is found to be around 600°C. It is found that sample with a relatively smooth surface has a mobility similar to the sample with lowest carrier concentration in spite of a relatively high carrier concentration, 730 cm 2 /V s, 2 × 10 19 cm -3 and 700 cm 2 /V s, 6 x 1018 cm -3 , showing that morphology is also one of the important factors to attain high electron mobility.

Fabrication of Freestanding GaN Wafers by Hydride Vapor‐Phase Epitaxy with Void‐Assisted Separation

Abstract: A novel technique for fabricating large-diameter freestanding GaN wafers has been developed. This technique uses a porous GaN template with a TiN nano-net on top as the starting substrate for hydride vapor-phase epitaxy (HVPE) growth. A mechanically weak layer containing numerous small voids is formed between the template and the thick GaN layer during HVPE growth. The thermal stress due to the difference in thermal expansion coefficients between the GaN and sapphire results in self-separation during the cooling process after HVPE growth. As a result, freestanding GaN wafers with a diameter of 45 mm are obtained. These wafers are crack-free and have a mirror-like surface. The full widths at half maximum of GaN (0002) and (1010) peaks in the X-ray rocking curve profile are narrow, i.e. 60 and 92 arcsec, respectively. The dislocation density is low, i.e. 5 × 10 6 cm -2 .

AlGaN Nanocolumns Grown by Molecular Beam Epitaxy: Optical and Structural Characterization

Abstract: High quality AlGaN nanocolumns have been grown by molecular beam epitaxy on Si(111) substrates. Scanning Electron Microscopy micrographs show hexagonal, single crystal columns with diameters in the range of 30 to 60 nm. The nominal Al content of the nanocolumns was changed from 16% to 40% by selecting the flux ratio between the Al and the total III-element, while keeping the growth temperature and the active nitrogen constant. The nominal values of the Al content are consistently lower than the experimental ones, most likely due to the high Ga desorption rates at the growth temperature. The Al composition trend versus the Al flux is consistent with the E 2 phonon energy values measured by inelastic light scattering. These results open the possibility to grow high quality low dimensional structures based on AlGaN/GaN/AlGaN heterocolumns for basic studies and device applications.

Failure Modes and Mechanisms of DC-Aged GaN LEDs

Abstract: This paper presents failure modes observed in long-term aging of high-brightness GaN/InGaN LEDs. The blue LEDs submitted to DC aging test present large decrease of emitted optical power and increase of diode reverse leakage current. Increase of parasitic series resistance, suggesting contact degradation, has also been found in stressed sample, together with apparent carrier density increases and reduction of the junction depletion width. Furthermore stressed LEDs present modification of a specific trap property: trap activation energy decreases from 340 meV in the virgin sample down to 75 meV in the stressed sample. Generation of non-radiative recombination centers seems to be one of the dominant failure mechanisms responsible for the observed electrical and optical LED degradations.

GaN boule growth: A pathway to GaN wafers with improved material quality

Abstract: Hydride vapor phase epitaxial growth of GaN boules was investigated. GaN boules were formed at growth rates in excess of 100 μm/h, resulting in boules as long as 5 mm in the c-axis direction. The boules were sliced into wafer blanks. Lapping, polishing and chemical-mechanical polishing was employed to produce wafers with a slightly stepped surface. Dislocations were decorated with hot phosphoric acid etching so that the change in defect density with boule length might be studied. Etch pit density correlated to threading dislocation density as measured by plan-view transmission electron microscopy and synchrotron white beam X-ray topography. The dislocation density of the GaN fabricated from the boules was as low as 10 4 cm -2 .

Synthesis and Magnetic Studies of Nanocrystalline Nickel Nitride Material

Abstract: Single domain Ni 3 N nitride particles are synthesized by simultaneous decomposition and nitridation in ammonia atmosphere of [Ni(NH 3 ) 6 ](NO 3 ) 2 complex crystals at 650 K. The Ni 3 N phase crystallizes in the hexagonal structure with unit cell parameters a = 4.624(4) A and c = 4.316(4) A, and has a crystallite size of 16 nm. In TEM study, these nanosize particles show spherical shape, and form particle aggregates of 18 nm size. Ni 3 N particles exhibit a Curie temperature of T C = 634 K. In the field dependence magnetic measurements, the presence of hysteresis loop indicates ferromagnetic behavior with σ s = 1.678 emu/g, σ R = 0.50 emu/g and μ 0 H c = 0.022 T. In this phase, the density of states (DOS) of Ni is dominated by 3d states and is mixed with 2p DOS of N. The contribution of d-electrons in the intra-band polarization affects the magnetic moment of Ni and thus the magnetic moment of Ni 3 N.

Bright, crack-free InGaN/GaN light emitters on Si(111)

Abstract: Crack-free, up to 2.8 μm thick GaN-based light emitting diodes were grown by metalorganic chemical vapor deposition on 2-inch Si(111) substrates. Elimination of cracks was achieved by using two ∼12 nm thick low-temperature AlN: Si interlayers for stress reduction. A significant enhancement in optical output power was obtained by an in situ insertion of a Si x N y mask. Transmission electron microscopy measurements showed a tenfold reduction in dislocation density to ∼10 9 cm -2 by the low-temperature AlN and Si x N y interlayers, resulting in a significant increase in luminescence intensity. Vertically contacted diodes showed a light output power of 152 μW at a current of 20 mA and a wavelength of 455 nm. Turn-on voltages around 2.8 V and series resistances of 55 Ω were obtained.

Infrared Reflectance and Electrical Conductivity of ?-Ga2O3

Abstract: Undoped β-Ga 2 O 3 single crystals were grown by the floating zone technique under several mixtures of N 2 and O 2 gas and 2 atm pressure. Polarized reflectance spectra in the 50-1200 cm -1 region showed all 12 IR active modes, with an additional background due to free carrier reflection. By least-squares fitting, the resonance frequency, the oscillator strength, the damping constant for each lattice oscillator, and the plasma frequency and damping factor of the free carriers were determined. Electrical measurements of the conductivity and the Hall-coefficient were correlated with the electrical parameters estimated from the optical data. The increase in the carrier concentration with decreasing oxygen partial pressure is accompanied by a decrease in carrier mobility, so that the conductivity does not vary significantly with the oxygen partial pressure.

Injection Dependence of the Electroluminescence Spectra of Phosphor Free GaN-Based White Light Emitting Diodes

Abstract: Multicolor, multi-quantum well light emitting diodes have been fabricated by molecular beam epitaxy by inserting quantum wells of various widths in the active region. The In content of the wells is 15%-20% and the color is governed by carrier confinement and the Stark effect. Combining a proper number of blue and yellow quantum wells allows to obtain monolithic white LEDs. The electroluminescence spectra of the diodes have been studied. At low injection, the luminescence intensity varies quadratically with the injection current, showing that the electroluminescence originates from the depleted region of the diode, and that non-radiative recombination paths exist. However, for higher injection currents, the luminescence efficiency of the wells situated near the n-side of the junction starts to vary linearly with the current, and this is accompanied by the appearance of GaN electroluminescence. We show that this is due to the entering of these wells into the neutral region of the diode, explaining the injection dependence of the color of these multicolor LEDs.

Single Chip White LEDs

Abstract: The operation principles of the various single chip luminescence conversion (LUCO).white LEDs, that can be realized with blue or ultraviolet (UV) LED chips, are briefly reviewed. In order to demonstrate the feasibility of the so far almost unexplored case of UV pumped tricolor LUCO white LEDs, In x Ga 1-x N single quantum well LED layer sequences were grown by MOCVD and processed into near-UV LED pump chips. Europium doped oxide and sulfide phosphors emitting in the blue, the green and the red spectral range were used for UV to visible conversion. With white blends prepared from these phosphors, housed white LEDs with high color quality were realized.

Electrical Conductivity in Mixed Calcium and Barium Iron Phosphate Glasses

Abstract: The electrical properties of xCaO-(20 - x)BaO-20Fe 2 O 3 -60P 2 O 5 glasses with x = 0, 5, 10, 15 and 20 mol% have been studied in the frequency range 400 Hz-200 kHz and over a temperature range 323-593 K. The ac conductivity as a function of temperature was divided into two domains, one where the absolute magnitude of the ac conductivity was close to the dc conductivity and another where it was larger than the dc conductivity. At the same temperature, the dc conductivity varied only slightly in glasses with different CaO and BaO content. It is concluded that the conductivity in these mixed CaO and BaO iron phosphate glasses is electronic and depends upon the Fe 2+ /(Fe 2+ + Fe 3+ ) ratio. In the high-temperature region above θ D /2 (where θ D is the Debye temperature) the Mott model of small polaron hopping (SPH) between nearest neighbours is consistent with the conductivity data, while at low temperature (below θ D ) the Greaves intermediate variable-range hopping (VRH) model is found to be appropriate. The electrical conduction of the glasses is confirmed to be non-adiabatic SPH. Mossbauer spectral analysis indicates the presence of both Fe 2+ and Fe 3+ ions.

Codoping Method for Solutions of Doping Problems in Wide-Band-Gap Semiconductors

Abstract: We propose a codoping method of using acceptors and donors simultaneously as a solution to the crucial doping problem, or unipolarity, of wide-band-gap semiconductors which exhibit an asymmetry in their ability to be doped as n-type or p-type. The deliberate codoping of donors is essential for the enhancement of acceptor incorporation, decrease of the binding energy of the acceptor impurity and stabilization of the ionic charge distributions in p-type highly doped semiconductors. The donor is not the p-type killer but a good by-player that activates acceptors, i.e., reactive codopant. Confirmation of the applicability of the codoping method to produce low-resistivity p-type GaN, ZnO, and ZnS was sought experimentally.

Anomalous slip - A feature of high-purity body-centred cubic metals

Abstract: The hitherto poorly understood phenomenon of anomalous slip in body-centred cubic (bcc) metal crystals, constituting a striking violation of Schmid's law of resolved shear stresses in limited ranges of temperature and crystallographic orientation of the stress axis, is shown to be a natural consequence of the fact that the slip planes of a 0 /2 screw dislocations change from {110} at low temperatures to {112} at higher temperatures. Above the temperature T of this change, which is responsible for the so-called lower bend of the flow-stress-temperature relationship, in the orientation range of anomalous slip the two slip systems with the largest resolved shear stress have different slip directions. The screw dislocations of these systems may avoid jog formation and generation of atomic defects by cross-slipping to the anomalous {110} plane. This mechanism accounts for the dependence of anomalous slip on temperature, crystallographic orientation, and purity. The variation from metal to metal is shown to be correlated with the elastic anisotropy. Mechanisms that might be responsible for this variation, in particular for the absence of anomalous slip in bcc iron, are discussed.

Nonlinear Behavior of Spontaneous and Piezoelectric Polarization in III–V Nitride Alloys

Abstract: We investigate spontaneous and piezoelectric polarization in wurtzite III-V nitride ternary alloys as a function of composition and microscopic structure, using ab initio density-functional techniques and Berry phase method. We find a strong nonlinear dependence of spontaneous polarization on composition in chemically disordered alloys. Nonlinearity is quite pronounced in those alloys (e.g. AlInN and InGaN) made of largely lattice-mismatched binaries. Spontaneous polarization bowing strongly depends on the microscopic structure of the alloy. Chemical ordering in the form of super-lattice structure may increase the bowing up to a factor of five. Piezoelectric polarization is also nonlinear, in random alloys the nonlinearity is entirely due to a nonlinear strain dependence on piezoelectric polarization in pure binary compounds, therefore piezoelectric coefficients follow Vegard's law. In chemically ordered InGaN and AlInN alloys piezoelectric coefficients deviate from Vegard's law, and this effect reduces the strength of the piezoelectric polarization up to 38% of its value in AlInN alloy.

On the Farsightedness (hyperopia) of the Standard k · p Model

Abstract: The use of a small number of bands in conventional k · p treatment of nanostructures leads to “farsightedness” (hyperopia), whereby the correct, detailed atomistic symmetry is not seen by the model, but only the global landscape symmetry is noted. Consequently, the real symmetry is confused with a higher symmetry. As a result, a number of important symmetry-mandated physical couplings are unwittingly set to zero in the k · p approach. These are often introduced, after-the-fact, “by hand”, via an ansatz. Sometimes physical effects (e.g., piezoelectricity) are invoked to fix the otherwise incorrect symmetry. Thus, whereas in atomistic theories of nanostructures (tight-binding, pseudopotentials) the physically correct symmetry is naturally forced upon us by the structure itself, in the standard k · p model it is accommodated ex post facto once it is known from sources outside the model itself.

Substituted ferrites studied by nuclear methods

Abstract: We report on recent investigations on substituted hexaferrites of the type SrM, Sr 1-x La x Fe 12-x Co x O 19 (0 ≤ x ≤ 0.4). It was already proved that the simultaneous substitution of Sr and Fe by La and Co, respectively, significantly improves the permanent magnetic properties of the material. In order to get a deeper insight into the magetic order from an atomistic point of view, 57 Fe Mossbauer spectroscopy and NMR ( 57 Fe, 59 Co) studies were carried out. It is demonstrated that by applying local probe hyperfine techniques the knowledge about this complex material can be considerably improved. The main results concern the preferred Co 2+ occupation on the 4f 2 and the 2a sites and the occurrence of Fe 2+ ions in the substituted material.

Low resistance Ti/Al/Mo/Au ohmic contacts for AlGaN/GaN heterostructure field effect transistors

Abstract: A metallization scheme consisting of Ti/Al/Mo/Au was utilized to develop low-resistance ohmic contacts to AlGaN/GaN heterostructure field effect transistors (HFET). A contact resistance as low as 0.20 Ωmm and a specific contact resistivity as low as 4.5 × 10 -7 Ωcm 2 were obtained using a pre-metallization surface treatment with SiCl 4 plasma at a self-bias voltage of -300 V in a reactive ion etching (RIE) system. X-ray photoelectron spectroscopy (XPS) measurements of the SiCl 4 plasma-treated surface revealed an increase in the N vacancies thereby increasing the donor concentration at the surface. Also a blue shift of the peak energy of the Ga 3d photoelectrons was observed showing that the Fermi level moved closer to the conduction band at the surface of the AlGaN.