1.3 lm GaAs=GaAsSb quantum well
laser grown by solid source molecular
beam epitaxy
P.-W. Liu, G.-H. Liao and H.-H. Lin
A highly strained GaAs=GaAs0.64Sb0.36single quantum well laser has
been grown on GaAs (100) substrate by using solid source molecular beam epitaxy. The uncoated broad-area laser demonstrates 1.292 mm pulsed operation with a low threshold current density of 300 A=cm2. The
spontaneous emission of the laser was also studied. The result reveals that the Auger recombination component dominates the threshold current at high temperature.
Introduction: 1.3 mm GaAs-based lasers are now of great interest because they have been recognised to be the key light sources for optical communications in the future. The most important advantage of 1.3 mm GaAs-based lasers is the capability to fabricate 1.3 mm vertical cavity surface emitting lasers (VCSELs) with the integration of the well-developed GaAs=AlAs distributed Bragg reflectors (DBRs) and AlAs oxidation techniques. One of the active media used for GaAs-based 1.3 mm lasers is the strained GaAs=GaAsSb quantum well (QW)[1–3]which exhibits a staggered type-II band alignment and possesses a potential of emitting photons with wavelength longer than that corresponding to the fundamental bandgap energy of each constituent. The strained GaAs=GaAsSb QW is a promising material for optical communication applications. We have successfully used molecular beam epitaxy (MBE) to grow a GaAs=GaAsSb double quantum well laser and obtained 1280 nm emission with a low threshold current density of 210 A=cm2at room temperature[3]. To extend the lasing wavelength to 1.3 mm, we increased the Sb content in the GaAsSb quantum well and reduced the number of quantum wells to one. In this Letter, we demonstrate the results of the grown single quantum well (SQW) laser. The spontaneous emission of the grown laser was also studied and compared with the reported results of the InGaAsN device[4].
Experiments: The lasers were grown on nþ
-GaAs (100) substrate by solid-source molecular beam epitaxy (SSMBE). The active region of the laser was grown at 500C. It consists of one 7 nm-thick GaAs0.64Sb0.36 quantum well and two 80 nm-thick GaAs barriers and was sandwiched between two 0.1 mm-thick AlGaAs step graded layers. The Al composition is graded from 0.1 to 0.5, centre to upper (lower) at the top (bottom) waveguide layer. Al0.6Ga0.4As layers with 1.5 mm thickness serve as the cladding layers of the laser. The AlGaAs layers and a 0.5 mm-thick buffer nþ-GaAs layer were grown at 580C. Finally, 50 mm-wide broad-area lasers with different cavity lengths were fabricated using standard photolithography, wet-etching, and metallisation processes.
After processing and mirror cleavage, the lasers were tested under 2 ms pulsed current with a repetition rate of 500 Hz. The lasing spectrums were taken using a HP70951A spectrum analyser. On the measurement of the spontaneous emission (SE) of the laser, we used a fibre bundle positioned at a fixed distance from the lateral side of the cavity. The collected SE signal was detected by a Ge detector using a lock-in technique.
Results and discussion: Fig. 1 shows the room temperature photo-luminescence (PL) spectrum of the GaAs=GaAsSb single quantum well laser with the top p-cladding layer removed. The peak position is at 1.3 mm, and the full width at half maximum (FWHM) is about 80 meV. The relatively broad FWHM is not unusual in GaAs=GaAsSb QWs[5–6], and can be attributed to the type-II inherent nature of the GaAs=GaAsSb QW and the Sb composition fluctuation in GaAsSb resulting from Sb segregation or phase separation[6–7].
Fig. 2shows the L-I characteristic of the laser device under pulsed-mode operation. The threshold current density is about 300 A=cm2. This is the lowest threshold current density reported for GaAs=GaAsSb quantum well lasers grown by MBE at 1.29–1.3 mm wavelength range. The inset inFig. 2shows the spectrum of a laser with a cavity length of 2.2 mm. The lasing wavelength is at 1.292 mm. We also measured the cavity length dependency of the threshold current density. The
threshold current density for infinite cavity length is 217 A=cm2for the GaAs=GaAsSb SQW laser. The internal quantum efficiency is 44% and the internal loss is 5.4 cm1. The low quantum efficiency may be due to the high carrier density in the active medium.
Fig. 1 Room temperature PL spectrum of GaAs=GaAsSb SQW laser with top p-cladding layer removed
Fig. 2 Light against current characteristic of 2.2 mm-long GaAs=GaAsSb SQW laser
Inset: Lasing spectra of GaAs=GaAsSb SQW laser
Because of the low gain and low spontaneous recombination rate in type-II QWs, carriers pile up in QWs and the waveguide layer. These high-density carriers may enhance the internal loss and degrade the external quantum efficiency especially in lasers with short cavity. The temperature characteristics were measured from 25 to 85C, and the characteristic temperature is 59 K. By measuring the spontaneous emission, we can obtain better insight into the recombination mechan-ism of the grown GaAs=GaAsSb SQW laser [4]. The integrated spontaneous emission rate L is proportional to n2, where n is the carrier density. The current flowing through the device I will be I / nz. The Z factor depicts the dominant current path in the laser[4]. Therefore, we have I / (L1=2)z[4]. By drawing a plot of ln(I) against ln(L1=2), the Z value can be derived.
Fig. 3plots the close-to-threshold power factor Zthagainst tempera-ture for the SQW GaAs=GaAsSb laser (squares) and for comparison the Zthvalues for a InGaAsN=GaAs laser[4](triangles). It shows that the temperature behaviour of the Zthvalue is close to that of the InGaAsN laser. The Zthvalue is about 2.2 near 300 K and increases to Zth2.7 at 353 K. This shows the significant increasing Auger current contribution at threshold as temperature increases. The high carrier density in type-II QWs as discussed above may account for the higher Auger recombina-tion rate. More detailed measurements and analysis are now in progress.
ELECTRONICS LETTERS
5th February 2004
Vol. 40
No. 3
Fig. 3 Power factor close to threshold Zthagainst temperature for GaAs=
GaAsSb SQW and InGaAsN=GaAs lasers
Conclusion: We have successfully grown a GaAs=GaAsSb single quantum well laser using SSMBE. The laser exhibits a low threshold current density of 300 A=cm2, and the lasing wavelength is at 1.292 mm. The internal quantum efficiency is 44% and the internal loss fitted is 5.4 cm1. The characteristic temperature is 59 K. The spontaneous emission of the laser was also measured. The higher carrier density in type-II QWs may result in a higher Auger recombi-nation rate which dominates the threshold current as temperature increases.
Acknowledgment: This work was supported by the National Science Council of the Republic of China under contract number NSC 92-2215-E-002-024.
#IEE 2004 2 December 2003
Electronics Letters online no: 20040119 doi: 10.1049/el:20040119
P.-W. Liu, G.-H. Liao and H.-H. Lin (Graduate Institute of Electronics Engineering and Department of Electrical Engineering, National Taiwan University, Room 419, No. 1, Section 4, Roosevelt Road, Taipei, Taiwan, Republic of China)
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