MOVPE growth of InAs quantum dots for mid-IR applications

TANG Xiao-hong¹, YIN Zong-you¹, DU An-yan², ZHAO Jing-hua¹, DENY S¹

1.Photonics Research Center, School of Electrical and Electronic Engineering

 Nanyang Technological University, 639798, Singapore;

2.Institute of Microelectronics, 11 Science Park Road, 117685, Singapore

Received 10 April 2006; accepted 25 April 2006

Abstract: InAs quantum dots (QDs) grown on In_xGa_1-xAs/InP matrix by low pressure metal organic vapor phase epitaxy (LP-MOVPE) in nitrogen ambient were studied. Formation of the InAs QDs with different growth conditions was investigated. To improve the dot size uniformity, a two-step growth method was used and investigated. It is found that morphology of the InAs QDs formed on such In_xGa_1-xAs/InP matrix is very sensitive to the growth conditions. InAs QDs with high density of 1.3×10¹⁰ cm^-2 are grown by using S-K growth method with fast growth rate. Using the two-step growth method, the InAs QDs size uniformity improves by 63% and 110% compared that of the dots grown by ordinary S-K method and ALE method, respectively. Narrow photoluminescence (PL) emission spectrum of the QDs grown by using the two-step growth method is received. FWHM of the PL curve is measured at 26 meV and the peak emission wavelength is larger than 2.3 μm at 77 K.

Key words: InAs; In_xGa_1-xAs/InP; quantum dots; MOVPE; mid-infrared

1 Introduction

Recently, quantum dots (QDs) have been used to develop high performance mid-infrared (IR) and 2-4 μm wavelength, lasers have attracted much interest[1-3]. Mid-IR lasers are attractive for application in analysis of pollutant and combustible gases, remote sensing and ranging, as well as the medical lasing surgery, etc. However, the longest emission wavelength from InAs QD structure reported only reaches near 2 μm at room temperature. InAs-rich ternary alloys such as InGaAs[4], InAlAs[5] and InAsP[6] were used to form larger QDs to extend the QDs’ emission wavelength. The longest light emission wavelength from these QDs structures was still limited at about 2 μm. By employing the lower energy bandgap binary and ternary III-V semiconductor alloys, InSb and InAsSb, to form QDs on InP substrate, the emission wavelength was pushed to 2.2 μm[7,8].

In this study, the extension of light emission wavelength of InAs QDs longer than 2 μm was studied by embedding the InAs QDs in In_xGa_1-xAs graded matrix layers.

All the samples were grown in a horizontal MOVPE reactor in Aixtron system. Low toxic MO sources tertiarybutylarsine (TBA) and tertiarybutyl-phosphine (TBP) were used as the group V precursors in the MOVPE growths. Trimethylindium (TMIn) and trimethylgallium (TMGa) were used as the group III sources. Purified N₂ was used as the carrier gas. The reactor pressure was set at 20 kPa for grown InAs QD, while for growing the other layers it was set at 10 kPa. On InP (001) substrate, a 200 nm InP buffer was grown at 600-630 ℃ after thermally annealing the substrate at 650 ℃ for 8 min. To extend the emission wavelength, the InAs QDs were embedded in the indium graded In_xGa_1-xAs layers. The indium content of the bottom graded In_xGa_1-xAs matrix layer of the QD structure was linearly increased from x = 0.53 (lattice matched to InP) to x=0.65-0.80. The graded In_xGa_1-xAs layers were grown at 600 ℃. The thickness of the graded matrix layer was 30 nm. The indium content of the In_{0.53→(0.53+y)}Ga_{0.47→ (0.47-y)}As graded layer was increased linearly from 0.53 to a target composition with different gradations y of 0, 0.12, 0.19 and 0.27, respectively. After that, the substrate temperature was lowered down to 550 ℃ for growing the InAs QDs. Morphology of the QDs was measured using an atomic force microscope (AFM). The PL measurements were performed by using a 532 nm diode laser as the exciting source. The PL signal from the sample was detected by a cooled PbS photodetector. Standard lock-in technique was used to amplify the signal before sending it to PC for processing.

3 Results and discussion

3.1 Morphology of QDs

The morphology and dot density of the InAs QDs of the samples were measured by atomic force microscopy (AFM). Fig.1 shows the 1 μm×1 μm AFM images of the InAs QDs grown by using different growth methods.

Sample (a) was grown by using conventional S-K method with fast growth rate, with the InAs growth rate of 0.83 mL/s. Sample (b) was grown by using atomic layer epitaxy growth with 8 repeated cycles. In each cycle, 40 mL/min(standard status) TMIn source flow was input for 2 s, followed by N₂ purge for 1 s and then 50 mL/min TBA flowed for 3 s. For sample (c), a two-step growth[9] was used. The worst uniformity of the dots was formed by the ALE growth. Sample (b) has the largest dot size fluctuation, which is defined as the largest diameter variation of the sample divided by the mean dot diameter, and the largest standard deviation of the QDs’ size. The QDs formed of sample (b) include dots and dashes. The QD density of sample (b) is as low

Fig.1 1 μm×1 μm AFM top mages of sample (a), (b) and (c), grown by conventional S-K fast growth, ALE and two-step growth (fast growth + ALE), respectively. Dot lateral size and height distributions are shown in the histograms

as 0.9×10¹⁰/cm². The QD density of sample (a) grown by conventional S-K method rate reaches 1.3×10¹⁰/cm² with almost no dash, but the dots formed are with large size dispersion. For sample (c), grown by the two-step (fast growth+ALE), the dot density formed is as high as 1.3×10¹⁰/cm² and at the sample time the dot size uniformity has also been improved much.

For one-step S-K fast growth of QDs, the nucleation rate is high when the deposited InAs wetting layer is thicker than its critical layer thickness because the indium atoms are forced to coalesce. At the same time, the short diffusion leads to form the nuclei with higher density but un-uniform[10]. It was reported that during the post-growth interruption, the reactor system was in a thermodynamic equilibrium growth, the edge barrier of the formed islands self-limited the further growth[11]. Although this growth could improve the QD size uniformity, the growth was mainly due to the diffusion of surface indium atoms. These atoms were limited. As a result, the finally formed QDs by one-step fast growth were still in large size dispersion[12].

There are three types of QDs formed in ALE grown sample (b) as shown in Fig.1(b). They are anisotropic nanodashes along [] direction, and dashes along other direction and dots. In ALE growth of InAs QDs, TMIn flux was set at low flow rate of 40 mL/min, the diffusion length of the adatoms is large. It has advantages in the anisotropic surface diffusion of In atoms along the [] direction, which results in the formation of the dashes along the [] direction. Formation of the dashes not along [] direction is caused by the unstable composition or strain of the graded In_0.53-0.72Ga_0.47-0.28As matrix layer. The InAs dots are formed between the nano-dashes in the ALE growth of InAs QDs. Under the ALE growth, indium and arsenic atoms were alternatively put into the reactor. Because of the edge barrier of the formed QDs, not all the indium atoms diffused to the dash matrix. Some surface atoms couldn’t reach the formed dashes. They would stay between the dashes and formed into dots. Since there are dashes and dots in sample (b), the QDs’ size uniformity is very bad and with low QD density.

Sample (c) was grown by using the two-step method in which the InAs QDs were grown with an initial fast growth + the ALE growth. The fast growth in step 1 growth has two advantages: suppressing the formation of dashes [13] and creating nuclei with high density after the InAs wetting layer growth. In step 2 ALE growth, larger dots grow slowly while the dots with smaller size grow fastly, so the QDs size uniformity is improved. The fluctuation of dots’ lateral size and height of sample (c) is 0.52 and 1.01, respectively, which is the smallest among the three samples.

3.2 Extension of emission wavelength

Fig.2 shows the 77 K normalized PL spectrum of the three InAs QDs samples. In sample C, InAs QDs are embedded between symmetric In_0.72-0.53Ga_0.28-0.47As graded layers; in sample D, the InAs QDs are embedded in the lattice matched In_0.53Ga_0.47As barriers; while for sample E, the InAs QDs are embedded in lower bandgap In_0.72Ga_0.28As barrier layers. The measured emission wavelengths of samples C, D and E are 2.14, 1.96 and 2.28 μm, respectively. The emission wavelength of QDs structure is determined by the bandgap of the QDs material, height of QDs[4,14], the energy bandgap of the barrier[15,16] and the strain of the cap layer[17,18]. The mean dot heights of the three samples as indicated in Table 1 are all around 9 nm. Large red-shift of the emission wavelength, ≥220 nm, of sample C and E is obtained compared with that of the sample D of QDs which is embedded in lattice match In_0.53Ga_0.47As barriers. The red shift of the samples’ emission peaks is mainly because of their barrier confinement and the cap layer’s strain. Sample E has the lowest barrier height and smallest strain of the In_0.72Ga_0.28As cap layer. In sample C, the InAs QDs are embedded in symmetric In_xGa_1-xAs graded barriers. It’s cap layer has less strain compared with that of sample D. At 77 K, the measured PL peaks of sample C and E are larger than 2.2 μm. This is the longest emission wavelength of InAs QDs reported so far, to our best knowledge. Fig.3 plots the PL emission intensity and full width of half maximum (FWHM) of the three QD structures. It shows that the InAs QDs formed on graded In_xGa_1-xAs barrier has similarly high PL emission intensity and the FWHM that the InAs QDs is embedded in high energy bandgap lattice-matched In_0.53Ga_0.47As barriers. Sample E, in which the InAs QDs

Fig.2 Photoluminescence spectrum of InAs QDs grown on different In_xGa_1-xAs matrixes measured at 77 K

Fig.3 PL emission intensity and FWHM of  samples C, D and E: 1—PL intensity; 2—FWHM

is embedded between the low energy bandgap In_0.72Ga_0.28As barriers, has the narrowest FWHM PL spectrum of 120 nm, but its emission intensity is the lowest. The narrower PL spectrum of sample E is attributed to its more uniform dot size. Its lower emission intensity is due to the lower energy barrier height. The confinement of the carriers in the QDs is weaker in sample E.

Table 1 Measured QD parameters of samples

High quality InAs QDs are grown on InP substrate by MOCVD using TBA and TBP as group V sources in pure nitrogen ambient. The QD density reaches 1.3×10¹⁰/cm² with greatly optimized size uniformity. By using a two-step (fast growth + ALE) growth, the QD density and size uniformity improve much. The emission wavelength of InAs QDs is red-shifted by growing the QDs on In_xGa_1-xAs matrix layers. Compared with the QDs grown on In_0.53Ga_0.47As lattice matched matrix, those grown on strained In_xGa_1-xAs matrix have higher QD density. PL emission wavelength of InAs/In_xGa_1-xAs/InP QD structure is red-shifted from 180 nm to 320 nm, compared with that of InAs/In_0.53Ga_0.47As/InP QDs. The longest wavelength, reported so far, of 2.28 μm at 77 K with narrow FWHM of 26 meV from InAs QDs is obtained.

Acknowledgements

The authors would thank Ms GEOK N B from the ion-beam laboratory of Nanyang Technological University for helping on AFM measurement.

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(Edited by CHEN Can-hua)

Corresponding author: TANG Xiao-hong; Tel: 65-67904438; E-mail: exhtang@ntu.edu.sg