InN/GaN heterojunction electrical behavior

Indium nitride (InN) has been the subject of intense research in recent years. Some of its most attractive features are its excellent transport properties such as its small band edge electron effective mass, high electron mobilities and peak drift velocities, and high frequency transient drift velocity oscillations [1]. These suggest enormous potential applications for InN in high frequency electronic devices. But to date the high unintentional bulk electron concentration (n~1018 cm-3) of undoped InN samples and the surface electron accumulation layer make it a hard task to create a reliable metalsemiconductor Schottky barrier. Some attempts have been made to overcome this problem by means of material oxidation [2] or deposition of insulators [3]. In this work we present a way to obtain an electrical rectification behaviour by means of heterojunction growth. Due to the big band gap differences among nitride semiconductors, it’s possible to create a structure with high band offsets. In InN/GaN heterojunctions, depending on the GaN doping, the magnitude of conduction and valence band offset are critical parameters which allow distinguishing among different electrical behaviours. The earliest estimate of the valence band offset at an InN–GaN heterojunction in a wurtzite structure was measured to be ~0.85 eV [4], while the Schottky barrier heights were determined to be ~ 1,4 eV [5].We grew In-face InN layer with varying thickness (between 150 nm and 1 mm) by plasma assisted molecular beam epitaxy (PA-MBE) on GaNntemplates (GaN/Al2O3), with temperatures ranging between 300°C and 450°C. The different doping in GaN template (Si doping, Fe doping and Mg doping) results in differences in band alignments of the two semiconductors changing electrical barriers for carriers and consequently electrical conduction behaviour. The processing of the devices includes metallization of the ohmic contacts on InN and GaN, for which we used Ti/Al/Ni/Au. Whereas an ohmic contact on InN is straightforward, the main issue was the fabrication of the contact on GaN due to the very low decomposition temperature of InN. A standard ohmic contact on GaN is generally obtained by high temperature rapid thermal annealing (RTA), typically done between 500ºC and 900ºC[6]. In this case, the limitation due to the presence of In-face InN imposes an upper limit on the temperature for the thermal annealing process and ohmic contact formation of about 450°C. We will present results on the morphology of the InN layers by X-Ray diffraction and SEM, and electrical measurements, in particular current-voltage and capacitance-voltage characteristics.

Free-standing urethane/urea elastomer films undoped and doped with ferro-nano-particles

We report on an experimental study of the structures presented by urethane/urea elastomeric films without and with ferromagnetic nanoparticles incorporated. The study is made by using the X-ray diffraction, nuclear magnetic resonance (NMR), optical, atomic and magnetic force (MFM) microscopy techniques, and mechanical assays. The structure of the elastomeric matrix is characterized by a distance of 0.46 nm between neighboring molecular segments, almost independent on the stretching applied. The shear casting performed in order to obtain the elastomeric films tends to orient the molecules parallel to the flow direction thus introducing anisotropy in the molecular network which is reflected on the values obtained for the orientational order parameter and its increase for the stretched films. In the case of nanoparticles-doped samples, the structure remains nearly unchanged although the local order parameter is clearly larger for the undoped films. NMR experiments evidence modifications in the molecular network local ordering. Micrometer size clusters were observed by MFM for even small concentration of magnetic particles.

Antiferromagnetism with spin polarization of GaN-based diluted magnetic semiconductors

The diluted magnetic semiconductors are promising materials for spintronic applications. Usually one intents to find the ferromagnetic state but recently the antiferromagnetism (AFM) was proposed to have some advantages. In this work, we verify the possibility to obtain spin polarization with an AFM state. In particular, we studied GaN 5% double doped with two different transition metals atoms (Mn and Co or Cr and Ni), forming the Mn(x)Co(0.056-x)Ga(0.944)N and Cr(x)Ni(0.056-x)Ga(0.944)N quaternary alloys. In order to simulate these systems in a more realistic way, and take into account composition fluctuations, we adapted the generalized quasichemical approach to diluted alloys, which is used in combination with spin density-functional theory. We find that is possible to obtain an AFM ground state up to 70% spin polarization.

Optical properties and transport in PLD-GaN

We present structural, optical and transport data on GaN samples grown by hybrid, two-step low temperature pulsed laser deposition. The band gap of samples with good crystallinity has been deduced from optical spectra. Large below gap band tails were observed. In samples with the lowest crystalline quality the PL spectra are quite dependent on spot laser incidence. The most intense PL lines can be attributed to excitons bounded to stacking faults. When the crystalline quality of the samples is increased the ubiquitous yellow emission band can be detected following a quenching process described by a similar activation energy to that one found in MOCVD grown samples. The samples with the highest quality present, besides the yellow band, show a large near band edge emission which peaked at 3.47 eV and could be observed up to room temperature. The large width of the NBE is attributed to effect of a wide distribution of band tail states on the excitons. Photoconductivity data supports this interpretation.

Elektrische Charakterisierung von GaN-basierten Halbleitersystemen

Galliumnitrid, elektrische und photoelektrische Eigenschaften, Heterostrukturen, Störstellen, Defekte, Dotierung, Ionenimplantation, Schottky-Raumladung

Growth of semi-polar GaN on high index silicon (11h) substrates by metal organic vapor phase epitaxy

Magdeburg, Univ., Fak. für Naturwiss., Diss., 2014

Wachstumsoptimierung und Charakterisierung von MOVPE-basierten GaN Pufferstrukturen auf Si(111) Substraten

Magdeburg, Univ., Fak. für Naturwiss., Diss., 2014

The effect of substrate on high-temperature annealing of GaN epilayers: Si versus sapphire

We have studied the effects of rapid thermal annealing at 1300¿°C on GaN epilayers grown on AlN buffered Si(111) and on sapphire substrates. After annealing, the epilayers grown on Si display visible alterations with craterlike morphology scattered over the surface. The annealed GaN/Si layers were characterized by a range of experimental techniques: scanning electron microscopy, optical confocal imaging, energy dispersive x-ray microanalysis, Raman scattering, and cathodoluminescence. A substantial Si migration to the GaN epilayer was observed in the crater regions, where decomposition of GaN and formation of Si3N4 crystallites as well as metallic Ga droplets and Si nanocrystals have occurred. The average diameter of the Si nanocrystals was estimated from Raman scattering to be around 3¿nm. Such annealing effects, which are not observed in GaN grown on sapphire, are a significant issue for applications of GaN grown on Si(111) substrates when subsequent high-temperature processing is required.

Nucleation and growth of GaN nanorods on Si (111) surfaces by plasma-assisted molecular beam epitaxy - The influence of Si- and Mg-doping

The self-assembled growth of GaN nanorods on Si (111) substrates by plasma-assisted molecular beam epitaxy under nitrogen-rich conditions is investigated. An amorphous silicon nitride layer is formed in the initial stage of growth that prevents the formation of a GaN wetting layer. The nucleation time was found to be strongly influenced by the substrate temperature and was more than 30 min for the applied growth conditions. The observed tapering and reduced length of silicon-doped nanorods is explained by enhanced nucleation on nonpolar facets and proves Ga-adatom diffusion on nanorod sidewalls as one contribution to the axial growth. The presence of Mg leads to an increased radial growth rate with a simultaneous decrease of the nanorod length and reduces the nucleation time for high Mg concentrations.

MOCVD growth andproperties of GaN and AlGaN layers

In this work GaN and AlGaN layers were grown by metal-organic chemical vapor deposition (MOCVD) on sapphire substrates. The research was carried out at Micro and Nanoscience Laboratory of Helsinki University of Technology. The objective of this thesis is the study of MOCVD technique for the growth of GaN and AlGaN films and optimization of growth parameters in purpose to improve crystal quality of the films. The widely used two-step and the new multistep methods have been used for GaN, AlGaN MOCVD growth on c-plane sapphire. Properties of the GaN and AlGaN layers were studied using in-situ reflectance monitoring during MOCVD growth, atomic force microscopy and x-ray diffraction. Compared to the two step method, the multistep method has produced even better qualities of the GaN and AlGaN layers and significant reduction of threading dislocation density.

Efeitos da adição de átomos de Mn na rede do GaN via métodos de estrutura eletrônica

A computational method to simulate the changes in the electronic structure of Ga1-xMn xN was performed in order to improve the understanding of the indirect contribution of Mn atoms. This periodic quantum-mechanical method is based on density functional theory at B3LYP level. The electronic structures are compared with experimental data of the absorption edge of the GaMnN. It was observed that the indirect influence of Mn through the structural parameters can account for the main part of the band gap variation for materials in the diluted regime (x<0.08), and is still significant for higher compositions (x~0.18).

Positron measurements in Mg-doped GaN and development of digital Doppler broadening spectroscopy

In this work parameters of Mg-doped GaN samples were studied using positron annihilation spectroscopy and analyzed. It is shown that gallium vacancies exist in an unintentionally doped sample. Next, the sample with higher concentration of Mg and low growth temperature contains vacancy clusters. In case of low concentration of Mg the growth temperature does not affect the formation of defects. Analog electronics can be replaced by a modern digital device. While promising a high quantity of benefits, the performance of these digitizers requires thorough adjustment. A 14-bit two channel digitizer has been tested in order to achieve better performance than the one of a traditional analog setup, and the adjustment process is described. It has been shown that the digital device is unable to achieve better energy resolution, but it is quite close to the corresponding attribute of the available analog system, which had been used for measurements in Mg-doped GaN.

GaN- ja AlGaN-puolijohteiden pintaominaisuuksista ja niiden muokkauksesta

Yhdistepuolijohde galliumnitridi (GaN) on energiatehokkaiden valkoisten ledien päämateriaali. GaN on kestävä materiaali; sillä on muun muassa alhainen säteilyreagointi ja suuri jännitekestävyys, minkä vuoksi GaN soveltuu hyvin myös maanpuolustus- ja avaruussovelluksiin. Sen iso energia-aukko mahdollistaa materiaalin käytön suurteho- ja suurtaajuussovelluksissa, ja sen suuri lämpökapasiteetti ja lämmönjohtavuuskyky avaavat edelleen uusia mahdollisuuksia materiaalin käytölle muun muassa tehovahvistimissa mikroaaltotaajuudella. GaN:ä käytetään myös sinisten ja violettien ledien valmistuksessa. Kun yhdisteeseen lisätään alumiinia, saadaan alumiinigalliumnitridin (AlGaN) energia-aukkoa ja muita ominaisuuksia säädeltyä alumiinin määrää muuttamalla. AlGaN-pintojen ominaisuuksia on tutkittu suhteellisen vähän, vaikka niiden merkitys kasvaa jatkuvasti kehitettäessä nanoteknologian sovelluksia. AlGaN-pintojen tutkiminen on tärkeää, koska monissa sovelluksissa juuri pinnat ja rajapinnat ovat huomattavassa asemassa laitteiden toiminnan kannalta. Pinnoilla sijaitseva atomien epäjärjestys, kidevirheet ja epäpuhtausatomit sekä atomien kemiallinen sidosympäristö kiinnostavat tutkijoita. Myös erilaisten pintakäsittelyjen vaikutusten tunteminen edellä mainittuihin ominaisuuksiin on tärkeää hyvien pintojen valmistuksen saavuttamiseksi. Tämän pro gradu -tutkielman on tarkoitus tutustua III–V-puolijohteiden GaN:n ja AlGaN:n pintaominaisuuksiin ja niiden muokkaukseen materiaalifysiikan pintatieteen menetelmillä. Mg-piristeisille Al0,5Ga0,5N-näytteille tehdyt LEED- ja STM-mittaukset osoittavat, että AlGaN-pinnat tarjoavat hyvän lähtökohdan bulkille tyypillisten ominaisuuksien tutkimiseen. XPS- ja SR-PES-mittaukset osoittavat, että Mg-piristys vaikuttaa aktivoituvan noin 700 °C:n lämmityksessä, ja näytteet sisältävät aiemmissa tutkimuksissa havaitun vedyn lisäksi happea ja hiiltä, jotka vaikuttavat magnesiumin aktivoitumiseen ja siitä syntyvään aukkokonsentraatioon. LEED-, STM- ja resistiivisyysmittaukset tukevat tehtyjä XPS- ja SR-PES-mittauksia. Al0,5Ga0,5N-näytteen energia-aukossa sijaitsevien aktiivisten vastaanottajatilojen määritettiin sijaitsevan 200–600 meV valenssivyön maksimin yläpuolella riippuen jälkilämmityksen kestosta.