MBE growth and characterisation of InGaAs quantum dot lasers

MBE growth and characterisation of InGaAs quantum dot lasers

Materials Science and Engineering B75 (2000) 121 – 125 www.elsevier.com/locate/mseb MBE growth and characterisation of InGaAs quantum dot lasers Jen-...

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Materials Science and Engineering B75 (2000) 121 – 125 www.elsevier.com/locate/mseb

MBE growth and characterisation of InGaAs quantum dot lasers Jen-Inn Chyi * Department of Electrical Engineering, National Central Uni6ersity, Chung-Li 32054, Taiwan, ROC

Abstract High quality self-assembled InGaAs quantum dots have been formed on GaAs by molecular beam epitaxy via Stranski–Krastonov growth mode, and have been employed to produce quantum dot lasers with reasonably good properties. The effects of growth conditions, substrate misorientation, and doping in quantum dots on the characteristics of quantum dots and quantum dot lasers are presented. It has been shown that higher density of quantum dots is obtained under higher As flux because the diffusion length of Ga adatoms is reduced. Higher degree of substrate misorientation also leads to higher density of quantum dots since the kinks on the surface have similar effect on the diffusion of cations. It is also found that doping in the quantum dots plays an important role in the performance of quantum dot lasers. Room temperature continuous wave operation has been achieved on Be-doped quantum dot lasers. Under pulse operation, characteristic temperature as high as 121 K between 20 and 70°C has been obtained. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Quantum dots; Lasers; Molecular beam epitaxy

1. Introduction Due to the atomic-like density of states, quantum dot lasers are expected to provide superior characteristics compared to quantum well and quantum wire lasers [1]. Low threshold current density and weak temperature dependence are two of the most attractive features of these novel semiconductor lasers. Up to date, self-assembled quantum dots prepared by in situ epitaxial growth techniques exhibit the best characteristics. The threshold current and the characteristic temperature of quantum dot lasers have been improved significantly over the past 5 years because of the optimization of material growth and in-depth understanding of device physics. Self-assembled In0.5Ga0.5As quantum dot lasers grown by molecular beam epitaxy (MBE) with threshold current of 51 mA and characteristic temperature of 350 K at low temperature were reported by Kirstaedter et al. [2]. Subsequently, many groups are devoted to the development of room-temperature operation quantum dot lasers prepared by MBE or metalorganic chemical vapor deposition (MOCVD) [3–7]. Room temperature CW operation of quantum disk lasers grown by MOCVD was then reported by Tem* Tel.: +886-3-4258241; fax: + 886-3-4255830. E-mail address: [email protected] (J.-I. Chyi)

myo [8]. More recently, Maksimov et al. reported that a high characteristic temperature of 385 K can be extended up to 50°C, while it decreases significantly to 85 K above 50°C [9]. Since the characteristics of nanostructures are closely related to the processes of their formation, the control over the size, the density, the regularity, and the integrity of quantum dots is essential for the realization of high performance quantum dot lasers. As shown in many previous reports, the formation of the quantum dots is dependent upon the kinetic processes of the deposited adatoms on the growing surfaces [2–4]. Therefore, the structural and optical properties of the quantum dots are expected to be influenced by the surface misorientation. Tsatsul’nikov et al. studied InAs quantum dots formed on misoriented GaAs substrates [5]. Lubyshev et al. reported the optical properties of InGaAs dots grown on the high index planes of GaAs substrates [6]. It was found that the peak position, full width at half-maximum (FWHM), and integrated intensity of the photoluminescence from quantum dots are orientation dependent. In this paper, we present the effects of substrate tilt angle on the structural and optical properties of In0.5Ga0.5As self-assembled quantum dots. Quantum dot lasers prepared under different conditions are characterized and compared as well.

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2. MBE growth and characterization

Fig. 1. Atomic force micrograph of the MBE grown InGaAs quantum dots on 4°-off (100) GaAs substrate with (a) As shutter open and (b) As shutter closed before the growth.

Fig. 2. Atomic force micrograph of the MBE grown InGaAs quantum dots on 15°-off (100) GaAs substrate.

The quantum dot samples were grown on GaAs substrates by solid source molecular beam epitaxy. Besides exact (100)-oriented substrates, 4 and 15°–off toward (111)A substrates were also used for comparison. After oxide desorption, a 200 nm-thick GaAs buffer layer was grown in the step-flow regime, i.e. low V/III flux ratio at 580°C, which led to the (3 ×1) surface reconstruction. Then, the substrate temperature was lowered to 520°C for the deposition of InGaAs quantum dots. The In0.5Ga0.5As quantum dots were grown with interruptions corresponding to 0.5 monolayer (ML) of In0.5Ga0.5As and 5 s of As4 irradiation for each period. Shown in Fig. 1(a) is the atomic force microscopy (AFM) image of the quantum dots formed on 4°-off substrate. The size of these well-organized InGaAs quantum dots is about 25 nm in diameter and 3 nm in height. The central blurry area is caused by successive measurements. A lower density of quantum dots, as shown in Fig. 1(b), can be obtained by commencing the growth under an As-free condition, i.e. the As shutter remains closed during the cooling steps. The dot density was decreased from about 3.6 to 1.2× 1010 cm − 2 while the size remains about the same. This is attributed to the longer diffusion length at the growing surface for the group III adatoms under low As background conditions. However, the dots grown on the 15°-off substrate exhibit significantly different characteristics as compared with the dots on the 4°-off substrate. Fig. 2. shows the AFM images of the quantum dots formed on the 15°-off substrate after the deposition of 3.5 ML of InGaAs at a growth rate of 0.7 ML s − 1. The size of the dots on the 15°-off substrate is about 190 nm, which is much larger than that of the dots on the 4°-off substrate. It is believed that the smaller distance between the terrace edges on the 15°-off substrate limits the diffusion of the adatoms during the growth and a large number of nucleation sites was provided for the formation of three-dimensional islands. Therefore, these dots have larger size due to its energetically favorable surface configuration. Since their size is larger than the terrace, the dots overlap each other and lose their regularity. Shown in Fig. 3 are the temperature-dependent integrated Photoluminescence (PL) intensity curves for the quantum dots formed on 0, 4 and 15°-off substrates. Due to the stronger three dimensional carrier confinement in the 4°-off samples, it has the highest activation energy of 180 meV and the best thermal stability. On the other hand, there exits a thicker two-dimensional wetting layer in the (100) samples, the thermalized carriers can hop easily between dots. It thus leads to a lower activation energy of 80 meV. As for the

J.-I. Chyi / Materials Science and Engineering B75 (2000) 121–125

Fig. 3. Arrhenius plot of the photoluminescence wavelength-integrated intensity of the quantum dot structures grown on vicinal substrates.

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coated lasers driven at various currents. The spontaneous emission wavelength is blue-shifted as the bias current is increased. Stimulated emission occurs at an injection current as low as 47 mA for one of the samples. The emission wavelength is located at the high-energy side of the spontaneous emission as observed in most of the quantum dot lasers [1,7–10]. As the injection current is increased further above the threshold, the 15°-off samples exhibit several satellite emissions in contrast to the single emission peak for the other two types of samples. Higher dot-size variation and inter-dot overlapping in the former are thought to be the origins of the phenomenon. The presence of localized excitons due to composition fluctuation, similar to the case of InGaN lasers, may not be ruled out though [11]. As shown in Fig. 4, stimulated emission is observed at room temperature as the 0, 4, and 15°-off samples are driven at 73, 47, and 65 mA, respectively. The lower current needed for the 4°-off sample is believed to be resultant from better formation of the quantum dots as evidenced by AFM observations. Compared to the 4°-off sample, the quantum dots on the exact (100) surface have a thicker wetting layer, which causes leakage current and reduced gain of the laser. Although the 15°-off sample has the thinnest wetting layer, its threshold current is still worse than that of the 4°-off one. We attribute it to the greater overlapping and size variation of the quantum dots that adversely affect the performance of the laser. Fig. 5 shows the temperature-dependent threshold current under pulse operation between 20 and 300 K. At 20 K, the (100) and 15°-off samples exhibit threshold currents of 6.2, and 6 mA, respectively, while that for the 4°-off one is 12 mA. As the measurement temperature is increased, a decrease in threshold cur-

Fig. 4. Electroluminescence spectra for the quantum dot lasers on vicinal substrates at room temperature.

15°-off sample, an activation energy of 142 meV is obtained. Although the significant overlapping between dots enhances the carrier tunneling probability, the PL intensity might not be seriously affected since tunneling process is weakly temperature-dependent.

3. InGaAs quantum dot lasers Investigation on quantum dot lasers grown on 0, 4, and 15°-off substrates was carried out. The active region consists of five stacks of InGaAs quantum dots separated by 10 nm-thick GaAs barrier. Fig. 4 shows the room temperature EL spectra of 500 mm-long un-

Fig. 5. Temperature dependence of the threshold currents for the quantum dot lasers grown on vicinal substrates.

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Fig. 6. Differential quantum efficiency and threshold current as a function of cavity length for the self-assembled In0.5Ga0.5As quantum dot lasers with doped and undoped dots.

Fig. 7. Temperature dependence of the threshold currents of the self-assembled In0.5Ga0.5As quantum dot lasers with doped and undoped dots.

rent is observed for all three samples. Similar result has been reported by Zhukov et al. [9]. They proposed a non-equilibrium/equilibrium model to explain this socalled negative characteristic temperature phenomenon. According to this model, the lowest threshold current occurs at about the same temperature of 75 K for all these samples since their bandgap difference between the InGaAs quantum dots and the GaAs barrier is the same. Above this critical temperature, the threshold currents of these lasers start to increase with temperature. However, the rate is lower for the 4o-off lasers than those for the others. This suggests that better carrier confinement occurs in the 4°-off sample. The enhanced thermalization of carriers via tunneling and hopping among the dots and wetting layer renders the (100) and the 15°-off lasers more temperature-sensitive. It is therefore not surprising to obtain a higher characteristic temperature of 177 K for the 4°-off samples, compared to 113 and 100 K for the (100) and the 15°-off ones, respectively. Based on the results above, 4°-off substrates were chosen for the study of doping effect on the properties of quantum dot lasers. Three types of quantum dots, i.e. Be-doped (5× 1017 cm − 3) dots, undoped dots and Si-doped (5 ×1017 cm − 3) dots were investigated. Before the implementation of lasers, these quantum dots were characterized by AFM and PL. From the AFM images,

it is found that with the presence of either Si or Be dopants, the quantum dots tend to have larger size, higher density but worse uniformity. It seems that Be and Si atoms serve as anti-surfactants in the process of quantum dot formation. Low temperature PL spectra show that the luminescence intensities from the Bedoped dots and Si-doped dots are about 2.4 and 0.6 times that of the undoped dots, respectively. The enhanced PL intensity for the Be-doped dots is attributed to the increased radiative recombination rate resulting from the built-in holes in the quantum dots. Whereas, the lower PL intensity observed from the Si-doped dots can be attributed to the incorporation of nonradiative impurities during the deposition [12]. As for the PL FWHM, an increase in linewidth is observed for both Si- and Be-doped dots. Similar effect has been observed in doped multi-quantum well structures [13]. Size variation observed in the doped quantum dots might also contribute to this linewidth broadening. Fig. 6 shows the differential quantum efficiency and threshold current of these three lasers with different cavity lengths under pulsed operation (5 KHz, 10 ms) at room temperature. The threshold currents for the ntype, p-type and undoped quantum dots lasers with 500 mm cavity length are 142, 108 and 138 mA, respectively. The Be-doped quantum dot laser exhibits the lowest threshold current among these devices. This reduction of threshold current is a result of the majority hole effect as observed in MQW lasers [13]. The threshold current per dot layer and the slope efficiency of a 500 mm-long Be-doped laser are measured to be 22 mA and 0.18 W/A, respectively. The deduced internal quantum efficiency and internal loss for the Be-doped lasers are 36% and 4.2 cm − 1, respectively. The highest internal quantum efficiency observed in the Be-doped laser implies that Be-doped quantum dots have the largest material gain. This is in good agreement with the results of PL measurements. Since the Be-doped quantum dots are beneficial for reducing threshold current as well as series resistance, continuous wave operation can also be achieved on the Be-doped lasers (not shown). Shown in Fig. 7 is the result of a series of light-intensity measurements between 20–70°C. These 500 mmlong lasers show an exponential increase in threshold current with temperature. Characteristic temperatures (To) of 100 and 121 K are deduced for the Si- and Bedoped quantum dot lasers. As for the undoped laser, it exhibits a characteristic temperature as high as 125 K. The difference in To between these lasers can be attributed to the carrier confinement in the quantum dots. Both state-filling effect and quality of quantum dots could affect the carrier confinement. In fact, the linewidth broadening of PL spectrum also reflects this effect as our undoped quantum dots exhibit the narrowest PL linewidth of 59 meV at room temperature.

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Council of ROC under contract NSC 88-2215-E008002.

4. Conclusions In this work, we have systematically studied the characteristics of In0.5Ga0.5As quantum dots and lasers grown on GaAs substrates. The effects of As flux and substrate tilt angle are examined. State filling effect is also demonstrated in terms of PL and EL spectra. Quantum dot lasers grown on 4°-off substrate exhibit lower threshold current and higher characteristic temperature than those grown on 0 and 15°-off substrates. The thermal quench behavior revealed by the Arrhenius plot of the photoluminescence wavelength-integrated intensity confirms the good carrier confinement of the quantum dots grown on 4°-off substrate. Furthermore, we have also investigated the lasing characteristics of quantum dot lasers with different types of active regions. The undoped quantum dot laser exhibits a characteristic temperature as high as 125 K between 20 and 70°C. While the Be-doped quantum dot laser has a slightly lower characteristic temperature of 121 K, it has the lowest threshold current and can be operated in continuous wave mode at room temperature.

Acknowledgements This work was supported by the National Science

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