Jonas Lähnemann
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PhD thesis / dissertation
Humboldt Universität zu Berlin, July 2013

Luminescence of group-III-V nanowires containing heterostructures
The role of polytypism, polarization fields and carrier localization

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In this thesis, the spectral and spatial luminescence distribution of heterostructures in self-induced nanowires (NWs) is investigated by cathodoluminescence spectroscopy in a scanning electron microscope. This method is complemented by data from both continuous and time-resolved micro-photoluminescence measurements. Three different structures are considered: (i) GaAs NWs containing segments of the wurtzite (WZ) and zincblende (ZB) polytypes, (ii) GaN microcrystals overgrown on GaN NWs, and (iii) (In,Ga)N insertions embedded in GaN NWs.

The polytypism of GaAs NWs results in complex emission spectra. The observation of luminescence either exclusively at energies below the ZB band gap or also at higher energies is explained by differences in the distribution of ZB and WZ segment thicknesses. Measurements at room temperature suggest that the band gap of WZ GaAs is at least 55 meV larger than that of the ZB phase.

The luminescence spectra of the GaN microcrystals contain distinct emission lines associated with stacking faults (SFs). SFs essentially constitute ZB quantum wells of varying thickness in a WZ matrix and it is shown that their emission energy is dominated by the spontaneous polarization. Through a detailed statistical analysis of the emission energies of the different SF types, emission energies of 3.42, 3.35 and 3.29 eV are determined for the intrinsic (I1 and I2) as well as the extrinsic SFs, respectively. From the corresponding energy differences, an experimental value of -0.022C/m² is derived for the spontaneous polarization of GaN.

The importance of both carrier localization and the quantum confined Stark effect induced by the piezoelectric polarization is shown for the luminescence of (In,Ga)N insertions in GaN NWs. Not only localized excitons, but also electrons and holes individually localized at different potential minima contribute to the observed emission.

1. Introduction

2. Fundamental Aspects
    2.1. Crystal structures of III-V semiconductors
    2.2. Polarization fields in III-V semiconductors
        2.2.1. Spontaneous polarization
        2.2.2. Piezoelectric polarization
        2.2.3. Quantum-confined Stark effect
    2.3. Polytypism in III-V nanostructures
        2.3.1. SiC: Prime example for polytypism
        2.3.2. Stacking faults in wurtzite crystals
        2.3.3. Stacking faults as quantum wells
        2.3.4. A model system to determine the spontaneous polarization
        2.3.5. The controversy concerning the band gap of wurtzite GaAs
    2.4. Carrier localization in (In,Ga)N heterostructures

3. Luminescence spectroscopy
    3.1. Recombination mechanisms in semiconductors
    3.2. Micro-photoluminescence
    3.3. Cathodoluminescence
        3.3.1. Scanning electron microscopy
        3.3.2. Setup and conditions for cathodoluminescence measurements
        3.3.3. Characteristics of cathodoluminescence spectroscopy
        3.3.4. Cathodoluminescence quenching in GaN nanowires

4. Samples and experimental background
    4.1. Nanowire growth and overgrowth
        4.1.1. Ga-assisted GaAs nanowire growth
        4.1.2. Growth and overgrowth of GaN nanowires
        4.1.3. Growth of (In,Ga)N insertions in GaN nanowires
    4.2. Sample preparation
    4.3. Additional experimental methods

5. Polytypism and luminescence of GaAs nanowires
    5.1. Geometry and structure of polytypic GaAs nanowires
    5.2. Luminescence of polytypic GaAs nanowires
        5.2.1. Micro-photoluminescence of individual GaAs nanowires
        5.2.2. Cathodoluminescence of individual GaAs nanowires
    5.3. Band gap of wurtzite GaAs
    5.4. Qualitative model for polytypic nanowires
        5.4.1. Spontaneous polarization of wurtzite GaAs
    5.5. Conclusions

6. Stacking fault luminescence and the spontaneous polarization of GaN
    6.1. Stacking faults and their luminescence
        6.1.1. Structural confirmation of the presence of stacking faults
        6.1.2. Luminescence signature of stacking faults
        6.1.3. Quantum-confined Stark effect and screening of the polarization fields
        6.1.4. Statistical analysis of the emission related to stacking faults
    6.2. Determination of the spontaneous polarization
        6.2.1. Comparison to values reported in the literature
    6.3. Luminescence associated with thicker ZB segments
        6.3.1. Calculation of the emission energies
        6.3.2. Luminescence of ZB segments at room temperature
    6.4. Conclusions

7. Polarization fields and carrier localization in (In,Ga)N/GaN nanowires
    7.1. (In,Ga)N insertions separated by wide barriers
        7.1.1. Influence of defects on the luminescence of the nanowires
        7.1.2. Variation of the In content among insertions in a single nanowire
        7.1.3. Indications for the presence of localization centers
        7.1.4. Time-resolved luminescence of the nanowire ensemble
    7.2. (In,Ga)N insertions separated by a thin barrier
        7.2.1. Structure and composition of the (In,Ga)N insertions
        7.2.2. Luminescence of the nanowire ensemble at room temperature
        7.2.3. Time-resolved luminescence of the nanowire ensemble
        7.2.4. Cathodoluminescence of individual nanowires
        7.2.5. Photoluminescence of individual nanowires
    7.3. Conclusions

8. Summary and outlook

A. Light emitting diodes based on nanowire ensembles
    A.1. The LED structure
    A.2. Electron beam-induced current measurements
    A.3. Top-view cathodoluminescence maps
    A.4. Conclusions

B. Additional CL spectral line scans for the GaN microcrystals

C. Spontaneous polarization in the context of a point charge model

D. Poisson-Schrödinger calculations

E. List of investigated samples

F. Conditions for cathodoluminescence measurements



Humboldt Universität zu Berlin - Institut für Physik
Paul Drude Institute for Solid State Electronics (PDI)

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