Growth and characterization of dilute bismide GaAs based alloys for high efficiency infra red laser diodes
Ludewig, Peter
A lot of energy in today's optical communication is wasted due to the inefficiency of optoelectronic devices operating at the telecommunication wavelength of 1.55 µm. The novel Ga(AsBi) material system is very promising to address this as it could enable the fabrication of high efficiency IR photonic devices such as laser diodes and EAM.
In this work the growth of Ga(AsBi) and Ga(NAsBi) on GaAs substrates using MOVPE was investigated, and thereby MQW as well as bulk-like structures were deposited. Several growth parameters were varied systematically applying pulsed as well as continuous precursor flows. Structural analysis such as HR-XRD, AFM, SEM, (S)TEM of the crystals were performed and PL spectroscopy has been carried out. Furthermore, the first electrically pumped Ga(AsBi) containing laser diode was demonstrated.
The surface of the first Ga(AsBi) samples were covered by metallic droplets consisting either of Ga, Bi or both in phase separated droplets. Drastically reducing the amounts of TMBi offer and carefully adjusting the TBAs/TEGa ratio enabled the growth of droplet free samples with measurable Bi fractions applying pulsed as well as continuous precursor flow. Bi segregates to the surface and it was found that the Bi incorporation depends on the Bi surface coverage during growth. In the case the coverage is too small, Bi only floats at the surface and does not get incorporated. At higher amounts the Bi fraction scales with the surface coverage up to a certain maximum and when it becomes too high Bi droplets begin to form. The maximum Bi fraction was found to increase with decreasing growth temperature that was varied in the range of 350 °C to 475 °C. However, at lower temperatures more defects occur and the precursor decomposition is reduced. Thus, 375 °C and 400 °C were stated to be most suitable for the MOVPE growth of Ga(AsBi) so far and maximum Bi fractions of about 7% and 5% were realized, respectively. The incorporation efficiency also increases with the growth rate and is inversely proportional to the TBAs/TEGa ratio within a range around unity were droplet free growth of Ga(AsBi) is possible. What makes the optimization and investigation of the growth conditions more complicated is that the before mentioned parameters are not independent. For example, the presence of Bi or the not fully decomposed TMBi at the surface reduces the growth rate, which is a hint that it hinders either the decomposition of the TEGa or its approach to the surface. Furthermore, lowering the growth temperature reduces the decomposition of the precursors and, hence, has an impact on the optimum TBAs/TEGa and TMBi/V ratios and reduces the growth rate.
However, chemically homogenous Ga(AsBi) samples were realized and, if the subsequent layer was grown at temperatures as high as 625 °C, sharp hetero interfaces were found. The Bi floating at the surface acts as surfactant that quenches the unintentional C incorporation that usually occurs at the low applied temperatures in MOVPE and it reduces the point defect density. Hence, strong bandgap PL was found for samples that are grown in the regime at which the Bi saturation sets in. The PL peak fits perfectly to the prediction from theory with linewidths (FWHM) of about 80 to 90 meV that are related to the disorder in dilute bismides.
To investigate whether the dilute bismides are suitable for optoelectronic devices, broad area Ga(AsBi) QW lasers were fabricated with Bi fractions of 2.2% and 4.4%. Electrical injection lasing of dilute bismides was demonstrated for the first time on a Ga(AsBi0.022) SQW laser with (AlGa)As barriers and cladding layers that showed room temperature lasing operation. The lowest achieved threshold current density of Ith=1.0 kA/cm² at pulsed current injection is very promising for such a new material system, however about 80% of Ith is lost by non-radiative recombination through defects. For devices with 4.4% Bi lasing was only found at low temperatures up to 180 K showing the necessity of further improving the growth of Ga(AsBi), especially when increasing the Bi fraction.
For the growth of Ga(NAsBi) it was stated that at constant Bi fraction the N fraction can easily be controlled by the UDMHy supply. Samples containing up to 4% Bi and N were realized, however, it was not possible to observe room temperature PL from those structures. Hence, photo reflection measurements were carried out, showing that at constant Bi fraction the bandgap reduction due to N is about 140 meV/%N confirming that N and Bi act independently on the band structure of GaAs.
Philipps-Universität Marburg
Physics
https://doi.org/10.17192/z2015.0050
urn:nbn:de:hebis:04-z2015-00503
opus:5913
2014-06-23
monograph
GaNAsBi
Physics
Physik
ppn:354382314
verdünnte Bismide
Drei-Fünf-Halbleiter
Ein Großteil der Energie in der optischen Datenübertragung geht in Form von Wärme verloren, insbesondere auf Grund von ineffizienten optischen Bauelementen im Wellenlängenbereich von 1.55 µm. Der Einsatz des neuartigen Ga(AsBi) Halbleiters könnte die Realisierung von hocheffizienten optischen Bauelementen wie Laserdioden im infraroten Bereich ermöglichen und damit zu einer deutlichen Reduzierung dieser Verluste beitragen.
In dieser Arbeit wurde das Wachstum von Ga(AsBi) und Ga(NAsBi) auf GaAs Subtraten mittels metallorganischer Gasphasenepitaxie untersucht. Die abgeschiedenen Kristalle wurden strukturell mittels HR-XRD, AFM, SEM, (S)TEM und optisch mittels PL Spektroskopie charakterisiert. Darüber hinaus konnten die ersten auf Ga(AsBi) basierenden Laserdioden demonstriert werden.
Die Oberflächen der Ga(AsBi) Proben waren zunächst von metallischen Tropfen übersät, die entweder aus Ga oder Bi bestanden oder aus einer Phasenseparation der beiden entstanden waren. Eine deutliche Reduzierung des TMBi Angebots und eine Feinjustierung des TBAs/TEGa Verhältnisses ermöglichten die Abscheidung von tropfenfreien Schichten mit messbarem Bi Gehalt. Es wurde festgestellt, dass Bi während des Wachstums an der Oberflächen segregiert und der Bi Einbau im Wesentlichen von der Oberflächenbedeckung abhängt. Ist diese zu gering, schwimmt das Bi lediglich an der Oberfläche auf und wird nicht in den Kristall eingebaut. Erst mit steigender Oberflächenbelegung kann eine lineare Abhängigkeit des Bi Gehalts zum angebotenen Bi festgestellt werden, wobei der Bi Einbau auf Grund der Metastabilität des Ga(AsBi) bei einem bestimmten Wert sättigt, was dann zur Bildung der Tropfen an der Oberfläche führt. Der maximale Bi Gehalt kann erhöht werden wenn die Temperatur reduziert wird. Bei niedrigeren Temperaturen bilden sich allerdings vermehrt Kristalldefekte und die Zerlegung der Precursoren ist geringer, sodass sich Temperaturen von 375 °C und 400 °C als am geeignetsten für das Wachstum von Ga(AsBi) herausgestellt haben. Hier wurden bisher Bi Konzentrationen von bis zu 7% bzw. 5% erreicht. Die Einbaueffizienz steigt ebenfalls mit steigender Wachstumsrate und mit sinkendem TBAs Angebot im Bereich des TBAs/TEGa Verhältnisses, in dem tropfenfreies Ga(AsBi) Wachstum möglich ist. Die Schwierigkeit bei der Optimierung und Untersuchung der Wachstumsbedingungen liegt darin, dass die zuvor genannten Parameter nicht zwingend unabhängig voneinander sind. So sinkt beispielsweise die Wachstumsrate durch die Anwesenheit von Bi oder des nicht vollkommen zerlegten TMBi an der Oberfläche, was darauf hinweist, dass entweder die Zerlegung von TEGa reduziert wird oder dessen Anhaftung an die Oberfläche. Darüber hinaus ändern sich die Zerlegungsraten der Precursoren im betrachteten Temperaturbereich mit der Temperatur und damit auch die Wachstumsrate und die optimalen TMBi/V und TBAs/TEGa Verhältnisse.
Dennoch konnten chemisch homogene Ga(AsBi) Proben mit scharfen Hetero-Grenzflächen realisiert werden, sofern die auf das Ga(AsBi) folgende Schicht bei 625 °C abgeschieden wurde. Das aufschwimmende segregierte Bi agiert als Surfactant, was den unbeabsichtigten Einbau von C, der bei den tiefen Wachstumstemperaturen auftritt, unterdrückt und zusätzlich die Defektdichte im Kristall reduziert. Daher konnte insbesondere bei den Proben, die in dem Wachstumsregime indem die Bi Sättigung einsetzt abgeschieden wurden, eine starke Bandlücken PL beobachtet werden. Die Emissionswellenlänge zeigt dabei eine nahezu perfekte Übereinstimmung mit der Theorie. Die Linienbreiten liegen bei 80 bis 90 meV, was auf die Unordnung in verdünnten Bismiden zurückzuführen ist.
Um die Eignung von Ga(AsBi) für optoelektronische Bauelemente zu untersuchen wurden Breitstreifen Ga(AsBi) QW Laser mit Bi Konzentrationen von 2.2% und 4.4% hergestellt. Elektrisch injizierte Verstärkung bei diesem Materialsystem konnte zum ersten Mal anhand eines Ga(AsBi0.022) SQW Lasers mit (AlGa)As Barrieren gezeigt werden. Die Schwellstromdichte von Ith=1.0 kA/cm² bei Raumtemperatur unter gepulster Stromzuführung ist vielversprechend für ein derartiges neues Material. Allerdings gehen etwa 80% des Schwellstroms durch nichtstrahlende Rekombination aufgrund von Defekten im Ga(AsBi) verloren. Laserdioden mit 4.4% Bi funktionierten bislang nur bei Temperaturen unterhalb von 180 K. Dies zeigt die Notwendigkeit die Wachtsumsbedingungen von Ga(AsBi) weiter zu verbessern, insbesondere im Hinblick auf die angestrebten Bi Konzentrationen von mehr als 10%.
Beim Wachstum von Ga(NAsBi) wurde festgestellt, dass bei konstanter Bi Konzentration der N Gehalt einfach über das UDMHy Angebot kontrolliert werden kann. Proben mit bis zu 4% Bi und N wurden hergestellt. Photo-Reflektionsmessungen zeigten, dass bei konstantem Bi Gehalt die Bandlücke um etwa 140 meV/%N verringert wird was die Annahme, dass Bi und N unabhängig auf die Bandlücke von GaAs wirken, bestätigt.
ths
Prof. Dr.
Volz
Kerstin
Volz, Kerstin (Prof. Dr.)
dilute bismides
MOCVD-Verfahren
Physik
GaAsBi
Growth and characterization of dilute bismide GaAs based alloys for high efficiency infra red laser diodes
MOVPE
2015-02-02
Wachstum und Charakterisierung von verdünnt Bismut-haltigen GaAs basierten Verbindungen für hoch effiziente infrarot Laserdioden
Philipps-Universität Marburg
A lot of energy in today's optical communication is wasted due to the inefficiency of optoelectronic devices operating at the telecommunication wavelength of 1.55 µm. The novel Ga(AsBi) material system is very promising to address this as it could enable the fabrication of high efficiency IR photonic devices such as laser diodes and EAM.
In this work the growth of Ga(AsBi) and Ga(NAsBi) on GaAs substrates using MOVPE was investigated, and thereby MQW as well as bulk-like structures were deposited. Several growth parameters were varied systematically applying pulsed as well as continuous precursor flows. Structural analysis such as HR-XRD, AFM, SEM, (S)TEM of the crystals were performed and PL spectroscopy has been carried out. Furthermore, the first electrically pumped Ga(AsBi) containing laser diode was demonstrated.
The surface of the first Ga(AsBi) samples were covered by metallic droplets consisting either of Ga, Bi or both in phase separated droplets. Drastically reducing the amounts of TMBi offer and carefully adjusting the TBAs/TEGa ratio enabled the growth of droplet free samples with measurable Bi fractions applying pulsed as well as continuous precursor flow. Bi segregates to the surface and it was found that the Bi incorporation depends on the Bi surface coverage during growth. In the case the coverage is too small, Bi only floats at the surface and does not get incorporated. At higher amounts the Bi fraction scales with the surface coverage up to a certain maximum and when it becomes too high Bi droplets begin to form. The maximum Bi fraction was found to increase with decreasing growth temperature that was varied in the range of 350 °C to 475 °C. However, at lower temperatures more defects occur and the precursor decomposition is reduced. Thus, 375 °C and 400 °C were stated to be most suitable for the MOVPE growth of Ga(AsBi) so far and maximum Bi fractions of about 7% and 5% were realized, respectively. The incorporation efficiency also increases with the growth rate and is inversely proportional to the TBAs/TEGa ratio within a range around unity were droplet free growth of Ga(AsBi) is possible. What makes the optimization and investigation of the growth conditions more complicated is that the before mentioned parameters are not independent. For example, the presence of Bi or the not fully decomposed TMBi at the surface reduces the growth rate, which is a hint that it hinders either the decomposition of the TEGa or its approach to the surface. Furthermore, lowering the growth temperature reduces the decomposition of the precursors and, hence, has an impact on the optimum TBAs/TEGa and TMBi/V ratios and reduces the growth rate.
However, chemically homogenous Ga(AsBi) samples were realized and, if the subsequent layer was grown at temperatures as high as 625 °C, sharp hetero interfaces were found. The Bi floating at the surface acts as surfactant that quenches the unintentional C incorporation that usually occurs at the low applied temperatures in MOVPE and it reduces the point defect density. Hence, strong bandgap PL was found for samples that are grown in the regime at which the Bi saturation sets in. The PL peak fits perfectly to the prediction from theory with linewidths (FWHM) of about 80 to 90 meV that are related to the disorder in dilute bismides.
To investigate whether the dilute bismides are suitable for optoelectronic devices, broad area Ga(AsBi) QW lasers were fabricated with Bi fractions of 2.2% and 4.4%. Electrical injection lasing of dilute bismides was demonstrated for the first time on a Ga(AsBi0.022) SQW laser with (AlGa)As barriers and cladding layers that showed room temperature lasing operation. The lowest achieved threshold current density of Ith=1.0 kA/cm² at pulsed current injection is very promising for such a new material system, however about 80% of Ith is lost by non-radiative recombination through defects. For devices with 4.4% Bi lasing was only found at low temperatures up to 180 K showing the necessity of further improving the growth of Ga(AsBi), especially when increasing the Bi fraction.
For the growth of Ga(NAsBi) it was stated that at constant Bi fraction the N fraction can easily be controlled by the UDMHy supply. Samples containing up to 4% Bi and N were realized, however, it was not possible to observe room temperature PL from those structures. Hence, photo reflection measurements were carried out, showing that at constant Bi fraction the bandgap reduction due to N is about 140 meV/%N confirming that N and Bi act independently on the band structure of GaAs.
Laserdiode
https://doi.org/10.17192/z2015.0050
urn:nbn:de:hebis:04-z2015-00503
Epitaxie
Fachbereich Physik
application/pdf
Publikationsserver der Universitätsbibliothek Marburg
Universitätsbibliothek Marburg
https://archiv.ub.uni-marburg.de/diss/z2015/0050/cover.png
Physik
infra red laser diodes
Bismut
2014
Ludewig, Peter
Ludewig
Peter
doctoralThesis
English
opus:5913
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