MOVPE growth and characterization of Cr-doped GaN [Elektronische Ressource] / submitted by Yong Suk Cho

Yong Suk Cho

Informations

• Publié par: rheinisch-westfalischen_technischen_hochschule_-rwth-_aachen
• Publié le: 01 janvier 2007
• Nombre de lectures: 20
• Langue: English
• Poids de l'ouvrage: 50 Mo

Extrait

MOVPE Growth and Characterization of Cr-doped GaN

From the Faculty of Georesources and Materials Engineering of the
RWTH Aachen University

Submitted by

Yong Suk Cho, Master of Engineering

from Seoul, Republic of Korea

in respect of the academic degree of


Doctor of Engineering

Approved thesis

Advisors: Univ.-Prof. Dr.rer. nat. Hans Lüth
Univ.-Prof. Dr.rer. nat. Gernot Heger

Date of the oral examination: 07. December 2007



This thesis is available in electronic format on the university library’s website



Contents

CONTENTS -------------------------------------------------------------------------------- i
ABSTRACT (English) ---------------------------------------------------------------- iv
ABSTRACT (German) ---------------------------------------------------------------- v
LIST OF FIGURES ------------------------------------------------------------------- vi
LIST OF TABLES ------------------------------------------------------------------- xvii
I. INTRODUCTION ------------------------------------------------------------- 1
References --------------------------------------------------------- 4
II. THEORETICAL BACKGROUND ------------------------------------- 6
2.1 Basic Properties of III-Nitride Semiconductors --------------------------- 6
2.1.1 Crystal Structure of GaN -------------------------------------------------- 6
2.2 Principles of MOVPE for III-Nitride Semiconductors Growth -------- 8
2.2.1 Overview of MOVPE Growth Process ---------------------------------- 8
2.2.2 Substrates for III-Nitrides Growth -------------------------------------- 14
2.2.3 III-Nitrides Growth by MOVPE ---------------------------------------- 17
2.2.4 Precursors for GaN:Cr Growth by MOVPE --------------------------- 18
2.3 Theory of Dilute Magnetic Semiconductors (DMSs) -------------------- 20
References -------------------------------------------------------------------------- 26
III. EXPERIMENTAL AND CHARACTERIZATION
METHODS ---------------------------------------------------------------------- 29
3.1 Setup of the MOVPE System ------------------------------------------------ 29
3.1.1 Gas Handling Systems and Sources ------------------------------------ 30
3.1.2 The Reactor --------------------------------------------------- 32
3.1.3 Exhaust of Byproducts --------------------------------------------------- 34
3.2 in-situ Monitoring -------------------------------------------------------------- 34
3.2.1 in-situ Normalization Reflectometry ----------------------------------- 35
3.2.2 Pyrometry ------------------------------------------------------------------ 38
3.3 ex-situ Characterization Methods ------------------------------------------- 44
i3.3.1 Morphology Inspection --------------------------------------------------- 44
3.3.1.1 Optical Microscopy -------------------------------------------------- 44
3.3.1.2 Scanning Electron Microscopy (SEM) ---------------------------- 46
3.3.1.3 Atomic Force Microscopy (AFM) --------------------------------- 47
3.3.1.4 Uniformity Inspection ------------------------------ 50
3.3.2 Layer Characterization --------------------------------------------------- 51
3.3.2.1 Hall Effect Measurement -----------------------------------
3.3.2.2 Contactless Sheet Resistance Measurement ---------------------- 53
3.3.2.3 Secondary Ion Mass Spectrometry (SIMS) ----------------------- 53
3.3.2.4 Photoluminescence (PL) --------------------------------------------- 55
3.3.2.5 Raman Spectroscopy ------------------------ 58
3.3.2.6 X-ray Diffraction (XRD) -------------------------------------------- 62
3.3.2.7 Transmission Electron Microscopy (TEM) ----------------------- 65
3.3.2.8 Superconducting Quantum Interference Device (SQUID) ------ 67
References -------------------------------------------------------------------------- 72
IV. INFLUENCE OF CARRIER GAS ON GaN GROWTH ----- 76
4.1 Growth Mechanism ------------------------------------------------------------ 76
4.2 Structural Properties ------------------------------------------- 87
References -------------------------------------------------------------------------- 99
V. GROWTH OF GaN:Cr BY MOVPE ------------------------------- 101
5.1 Growth without Undoped GaN Buffer Layer --------------------------- 101
5.2 Different Hardware Setups at High Growth Temperature ---------- 104
5.3 Different Carrier Gases at High Growth Temperature --------------- 107
5.4 Growth at Different Temperatures ---------------------------------------- 111
5.5 Different Carrier Gases at Low Growth Temperature --------------- 131
5.6 Increasing Cr Concentration ----------------------------------------------- 138
5.7 Post Growth Annealing ------------------------------------------------------ 146
References ------------------------------------------ 153
iiVI. CONCLUSION AND OUTLOOK ----------------------------------- 155
ACKNOWLEDGMENTS
PUBLICATION LIST
CURRICULUM VITAE
iiiAbstract

In this thesis chromium (Cr) incorporation during GaN growth by metal organic vapor
phase epitaxy (MOVPE) was studied for the first time with the intention of producing a dilute
magnetic semiconductor (DMS) with room temperature ferromagnetism. To this end the
following growth parameters were investigated and optimized: carrier gas, precursor access and
ratios, and growth temperature as well as post growth annealing. Also the effect of different
H /N carrier gas ratios on the undoped GaN buffer layer was explored. The N content in the 2 2 2
carrier gas affected the growth mechanism of the GaN layer. The threading dislocation density
as well as the sheet resistance increases with N content in the carrier gas. Cr-doped GaN 2
(GaN:Cr) layers were grown on the optimised undoped GaN buffer layer. Uniform Cr
distribution, good surface morphology and room temperature ferromagnetism were to be
obtained. The inverted precursor access to the substrate promoted high Cr incorporation
efficiency. A certain amount of H in the carrier gas was necessary to obtain coalesced surface 2
morphology of GaN:Cr layers. The GaN:Cr layer grown at 950 °C exhibited smooth surface
morphology and enhanced Cr incorporation into the layer as well as ferromagnetic behaviour at
room temperature. The Curie temperature (T ) was determined to be about 620 K. A high Cr/Ga C
source ratio increases the Cr concentration in the GaN:Cr layers and also increases the thermo-
remanent magnetization. The use of post growth annealing at 700 °C is advantageous for the
magnetic properties.

ivAbstract

Diese Arbeit beschäftigt sich mit dem Einbau von Cr beim Wachstum von GaN mit
dem Ziel, einen verdünnten magnetischen Halbleiter herzustellen, der ferromagnetisch bei
Raumtemperatur ist. Zum ersten Mal wird das Wachstum in der metallorganischen
Gasphasenepitaxie durchgeführt und der Einfluss von Wachstumstemperatur, Gasatmosphäre,
Einlass der Quellenverbindungen sowie deren Partialdruckverhältnis auf Einbau und
Schichteigenschaften untersucht. Auch wird die Frage nach der Nützlichkeit einer undotierten
GaN-Pufferschicht und eines Temperschrittes nach der Epitaxie gestellt.
Zunächst wurde der Einfluss der Gasatmosphäre auf das Wachstum einer undotierten
GaN-Schicht untersucht. Der Wachstumsmechanismus wurde durch die Reaktionsatmosphäre
verändert und führt zu einer Erhöhung der Versetzungsdichte und des Schichtwiderstandes. Das
Ergebnis der Arbeit mit Hinblick auf Cr-Dotierung in GaN ist, dass die Schichten bei niedriger
Wachstumstemperatur in einer Gasatmosphäre, die Wasserstoff enthält, auf einer undotierten
Pufferschicht mit invertiertem Quellenverbindungseinlass am besten hergestellt werden, um
Koaleszenz in der Schicht zu erhalten. Die bei 950 °C hergestellte Schicht zeigte eine glatte
Morphologie und gesteigerten Cr-Einbau sowie Ferromagnetismus bei Raumtemperatur. Eine
Curie-Temperatur von 620 K wurde ermittelt. Ein noch höheres Cr/Ga-Verhältnis führt zu
gesteigerten Cr-Einbau und einer noch höheren Magnetisierung. Die Verwendung eines Temper-
Schrittes bei 700 °C unmittelbar nach der Epitaxie ist von Vorteil für die magnetischen
Eigenschaften.

vList of Figures

Figure 2.1. The comparison of (a) WZ and (b) ZB structure. ---------------------------------------- 7
Figure 2.2. Reaction steps in a MOVPE process. ---------------------------------------------------- 10
Figure 2.3. Qualitative temperature dependence of the growth rate for MOPVE process. ----- 11
Figure 2.4. Schematic illustration of the relation in crystallographic orientation between the
(0001) GaN layer ( ●) and the (0001) sapphire substrate ( ○) [6]. -------------------- 16
Figure 2.5. Growth model for GaN layer on sapphire substrate [3]. ------------------------------- 18
Figure 2.6. Predicted Curie temperatures as a function of bandgap with ZB structures, along
with some experimental results [20]. ----------------------------------------------------- 21
Figure 2.7. Predicted Curie temperatures as a function of the band gap. Computed values of the
Curie temperature (T ) for variou