Multiple Wavelength InGaAs Quantum Dot Lasers Using Ion Implantation Induced Intermixing
© to the authors 2007
Received: 27 August 2007
Accepted: 6 September 2007
Published: 25 September 2007
We demonstrate multiple wavelength InGaAs quantum dot lasers using ion implantation induced intermixing. Proton implantation, followed by annealing is used to create differential interdiffusion in the active region of the devices. The characteristics (lasing-spectra, threshold currents and slope efficiencies) of the multi-wavelength devices are compared to those of as-grown devices and the differences are explained in terms of altered energy level spacing in the annealed quantum dots.
Integration of several quantum dot (QD) devices on a single chip offers the advantages of compact size, high speed and low optical losses, added to the advantages discrete QD devices offer due to three-dimensional carrier confinement in the active region. Ion implantation induced intermixing is a technique that is compatible with planar processing and can be used for band gap tuning, essential for device integration. Ion implantation induced intermixing has been widely used for band gap tuning of quantum wells [1, 2] (QW) and QDs [3–8]. QW based integrated photonic devices have also been demonstrated using implantation induced intermixing [9, 10]. Though there are many reports on band gap tuning of QDs, there are no reports to date on multi-color QD lasers using ion implantation induced intermixing. In this letter, we report on multi-wavelength QD lasers fabricated using implantation induced intermixing. We first demonstrate differential band gap shift and effect on carrier confinement and energy level spacing in QDs due to ion implantation induced intermixing, using photoluminescence (PL). Then we report on multi-color QD lasers and discuss the effect of intermixing induced changes in confinement and energy level separation in the active region on the performance of the devices.
The thin p-clad laser structures studied in this work were grown by metal-organic chemical vapor deposition (MOCVD) system. Trimethylindium, trimethylgallium and AsH3 with H2 as the carrier gas were used as the precursor sources; Silane and CCl4 were used as n- and p-type dopant sources, respectively. The active region of the lasers consisted of five layers of In0.5Ga0.5As QDs incorporated into GaAs barrier layers. 100 keV protons at a dose of 5 × 1013 cm−2 were implanted into the active region, wherever mentioned. Following implantation, the device structures were annealed at 600 °C for 30 min in the presence of AsH3. Annealing conditions were chosen to maximize the room temperature (RT) PL recovery from the QDs in the active region. PL spectra from the active region of the devices were obtained prior to device fabrication by exciting the samples using a 635 nm laser and collecting the luminescence using a cooled InGaAs detector. Four micrometers wide ridge wave-guide lasers were fabricated from the annealed and as-grown laser structures using the standard device processing steps . The as-cleaved devices were tested at RT in pulsed mode (duty cycle 5%).
The carrier confinement in the active region of the devices is determined by the separation between the GaAs (barrier) band edge and the QD energy levels (E conf in Fig. 1). Assuming a conduction band offset of 0.6 E g, the confinement energy for the electrons occupying the lowest energy level in the conduction band of QDs in the as-grown device structure is ∼230 meV (not shown), whereas the confinement energy for the carriers occupying the GS of QDs in the un-implanted and implanted, annealed devices is ∼180 meV and 168 meV, respectively. Thermal population of QD ES depends on the energy separation between the GS and ES (ΔE in Fig. 1). The separation between consecutive energy levels in the conduction band calculated from the observed peaks in the PL spectra from both implanted (P1′ and P2′) and un-implanted (P1 and P2) samples is ∼35 meV, which is very close to the thermal energy of carriers at RT. These results indicate that the carriers in the annealed quantum dots have smaller confinement energy and greater probability of occupying ES.
Simultaneous lasing from different energy levels in QD or QW lasers has been observed by several groups [13–16], especially for short cavity lengths and was explained in terms of high cavity losses, which lead to increase in threshold and thus to increased band filling . Increasing the operation temperature would also require higher injection and leads to the same effect . At RT, the as-grown devices studied in this work lase from QD GS for all cavity lengths whereas the annealed devices show simultaneous lasing from different energy levels (for L ≤ 1.5 mm for un-implanted and L ≤ 2 mm for implanted devices). Inset in Fig. 2 shows the lasing spectra of 1 mm long devices fabricated from annealed device structures (with or without implantation). As will be discussed later, the annealed devices have higher thresholds (consequence of smaller E conf and ΔE) leading to increased band filling. Also, smaller values of ΔE in the annealed QDs increase the probability of thermal population of higher energy levels. So we attribute simultaneous lasing from different energy levels in the annealed devices to modification of energy level separation in the active region, as a consequence of annealing.
The above results indicate that smaller values of ΔE and E confin the multiple-wavelength devices result in poor performance compared to the as-grown devices. Improvement in device performance has been reported by engineering the QD energy levels  (to increase ΔE) and using AlGaAs barriers  (to increase E conf). We believe that using similar approaches with implantation induced intermixing may result in multi-color lasers with improved characteristics.
In summary, we have demonstrated multiple wavelength InGaAs QD lasers using ion implantation induced intermixing. For a cavity length of 2 mm, the shift in the lasing wavelength of un-implanted and implanted devices is 40 meV. Implantation followed by annealing of device structures used for achieving differential band gap shift alters the energy level spacing in the active-region, resulting in broader lasing spectra, higher threshold current densities and lower slope efficiencies with respect to the as-grown devices. Hence band gap tuning using ion implantation induced intermixing requires careful design of devices to minimize carrier loss from the QD active region.
Thanks are due to Marie Bruneau, Michael Aggett for expert technical advice. The Australian Research Council is gratefully acknowledged for the financial support.
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