三五族半導體量子點結構之物理與元件

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(3) Studies of Quantum Dot Structure and Devices on III-V Compound Semiconductors .

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(11) Ide^u% vwxyYzO\!S { I|}|~ N ade # `| z!S QR%@!!"O\ UV<!"@ !%[AB!<T 0 HIJ KL;<¡¢£ ¤% ¥ ¦!"¦§¨© Abstract. was verified by atomic force microscope (AFM) images and the shift of photoluminescence (PL). The PL emission from the quantum dots was very strong and the full-width at half-maximum (FWHM) of the emission peak was as small as 7.7 meV(Fig.1), indicating excellent quality and very uniform dot formation. The effect of nominal thickness on the quantum dot formation has been investigated.(Fig.2) We also report photoluminescence measurements on In0.25Ga0.75As/GaAs quantum well and dots grown on (111)B GaAs substrate in high magnetic fields up to 45 Tesla. A well-defined PL line with full width at half maximum of approximately 5.5 meV is observed. From an analysis of the zero field transition energy, we point out the importance of an internal piezoelectric field. By analyzing of the diamagnetic shift of the PL in both Faraday and Voigt configuration, the optical characteristics of a quasi-zero dimensional exciton are discussed. Besides, we report a magneto optical characterization of the InGaAs/GaAs quantum well (QW) and quantum dot (QD) structures grown on (111)B GaAs substrate. The photoluminescence (PL) peak shift at high excitation condition is used in distinguishing QW from QD structure in addition to the atomic force microscopy (AFM) image. The binding energy in InGaAs/GaAs QW in (111)B direction is about 1 meV. The extent of the wave function obtained from the diamagnetic shift of the PL peak energy is consistent with the result. We present a simple in situ method to fabricate high-quality InGaAs/GaAs quantum dots on (111)B GaAs substrates. The mechanism of the quantum dot formation is not strain relaxation but the growth characteristic of (111)B GaAs under low substrate temperatures. When the growth is performed at low temperatures, the layer-by-layer growth mode is replaced by island growth and therefore quantum dots are formed. The formation of the quantum dots 1.

(12) QD and the lateral sizes and the heights of the dots could also be estimated from the spatial extents of the carrier wave functions (ECWFs).. calculated by the k·p method. The InGaAs/GaAs QD lateral confine energy and the dots size are also estimated from diamagnetic shift of the PL lines. The lateral confine energy is 6 meV. The mean radii of the InGaAs QD is about 17 nm and the dot height is about 7 nm, which is in good agreement with the result as revealed in AFM image.. It is known that epitaxial growth on GaAs (111)B substrates is difficult.[1] In order to obtain layer-by-layer growth by molecular beam epitaxy (MBE), the substrate Keywords: Research Project, Report Style, temperature and the V/III ratio should be National Science Council maintained at a certain level so that the.

(13) surface reconstruction is √19×√19.[2] If the substrate temperature drops below 500, Because of the advances in the surface reconstruction changes to 2×2 and semiconductor epitaxial technology and the growth results in pyramids and twins.[3] process technology, devices based on This growth characteristic, however, was quantum phenomena have recently been found to be useful in quantum dot formation. extensively explored. Due to the small It has been reported that the 2×2 surface effective mass and high dielectric constant, reconstruction is an As rich condition with the quantum mechanical effects, which are many As trimers on top of the As surface normally seen in the atomic scale, are easily layer.[4] During growth, it is difficult for observable in semiconductor structures with incoming Ga atoms to bond to surface As dimensions of tens or hundreds of angstroms. atoms by breaking the connection between The quantum mechanical effects combined As trimers and the surface atoms.[5] with the optical properties of III-V Therefore, layer-by-layer growth is hindered heterostructures open up a whole new area and island growth is promoted. Therefore, if for devices applications. Just to name a few: the growth proceeds in this manner, islands quantum well lasers, quantum well infrared are formed. In this study, we found that detectors, and quantum well modulators. when a very thin layer is grown under 2×2 These devices have all been proved to be surface reconstruction, small coherent useful or potential useful for future electronic quantum-size islands are formed. systems. One of the most commonly used Magneto-optics in high magnetic fields methods in quantum dot fabrication is the is a powerful tool to investigate the electronic so-called "self-assembled dot" (SAD) method states in low-dimensional system. We could using the Stranski-Krastanov growth mode. perform the magneto-photoluminescence (PL) This method utilizes strain relaxation measurement on InGaAs/GaAs QW and QD between two materials with large lattice structures on (111)B substrate in magnetic mismatch to facilitate island growth and fields up to 45 Tesla. From the high therefore quantum dots formation. excitation-intensity PL spectra, the structure Experimental results of SAD on substrates of of the sample could be categorized as QW or (100) and (111)B GaAs have been 2.

(14) 480 oC is wider but its height is thinner. The lateral size becomes narrower but the height becomes thicker as the growing temperature is lowered to 450 oC.. reported.[6-8] However, the control of growth condition is critical for good uniformity of quantum dots. In our work, the mechanism of the quantum dot formation is not strain relaxation but the growth characteristic of (111)B GaAs under low substrate temperatures. We found that when the growth is performed at low temperatures, the layer-by-layer growth mode is replaced by island growth. The formation of the quantum dots was verified by atomic force microscopy (AFM) images and the shift of photoluminescence (PL). The full-width at half-maximum (FWHM) of the emission peak of the dots was as small as 7.7 meV, indicating very uniform dot formation. In the magneto- PL measurements, InGaAs/GaAs QWs and QDs grown on (111)B GaAs substrate by varying the o growing temperature from 525 to 450 C were used. We have used magneto-optics in high magnetic fields to investigate the InGaAs/GaAs QWs and QDs grown on (111)B GaAs substrate at different growing temperature. The InGaAs/GaAs QW binding energy can be obtained by the diamagnetic shift and is about 1 meV. In addition, the ECWFs are also deduced from the diamagnetic shift. The inplane ECWFs changed from 18.7 to 16.7 nm when the growing temperature varied from 525 to 450 o C. The inplane ECWFs in QDs are slightly smaller than that in QWs because the weak confinement of the wider lateral size of the dots. The ECWFs in QWs in the growth direction obtained by diamagnetic shift are consistent with which calculated by k.p theory. The ECWFs in the z direction decreased from 525 to 480 oC and reached the minimum at 480 oC, the temperature which QD structure began to form, then increased as temperature goes down to 450 oC. The lateral size of the dot formed at growing temperature.

(15) In our research, optical characteristics of InGaAs/GaAs quantum dots on (111)B GaAs substrates fabricated by a new method were studied by photoluminescence. Island formation was clearly shown by AFM images. A PL linewidth of 7.7 meV at 8 K is obtained, which is the narrowest reported for quantum dots. The quality was excellent when the nominal thickness was not greater than 2 nm and the optimal growth temperature of InGaAs quantum dots was found to be between 480 and 465. The success of high quality quantum dots fabrication and the study of the optimal growth condition are very helpful for the device applications of quantum dots. The result has been published in Japanese Journal of Applied Physics[9]. We have also used magneto-optics in high magnetic fields to investigate the InGaAs/GaAs QWs and QDs grown on (111)B GaAs substrate at different growing temperature. The structures show a large diamagnetic shift of the energy levels, and measurements of the anisotropy of the shift allow us to conclude that there is enhanced confinement in the dots. The ECWFs are also deduced from the diamagnetic shift. The inplane ECWFs changed from 18.7 to 16.7 nm when the growing temperature varied from 525 to 450 oC. These measurements support that high quality quantum dots are formed and help us to understand the physics of quantum dots. The results are also publlished or submitted[10,11]. 3.

(16) . [7]. [1] [2]. [8]. [10]. [11]. 1/118. 1/116. B1. B2. !)222*C !)211*. o. 5 25 C. 1/117. 1/115. PL Inte nsit y (arb. units). [6]. [9]. 1/114. 1/113. 1/112. 1/111 911. 1/1136. 931. 951. 971. W a v e le n gt h ( n m ) (a). B3. 991. !)211*. 1/116 1/115 1/114 1/113. 1/111 911. :11. 9 31. 951. 971. W av e le ng th ( nm ). 991. :11. ( b). 1/1123. !)222*C. o. 4 80 C. !)211*. ! ) 2 2 2* C. B4. ! ) 2 1 1*. o. 1/1121. 1/1131. 1/1126. 1/1121. 1/1116. 1/1111 911. !)222*C. o. 510 C. 1/112. P L Intensity ( ar b. units). [5]. Pl Inte nsit y (arb. units). [4]. PL Int ensity ( arb. units). [3]. A. Y. Cho: J. Appl. Phys. 41 (1970) 2780. P. Chen, K. C. Rajkumar and A. Madhukar: Appl. Phys. Lett. 58 (1991) 1771. P. Chen, K. C. Rajkumar and A. Madhukar: J. Vac. Sci. Technol. B 9 (1991) 2312. D. K. Bjegelsen, R. D. Bringans, J. E. Northrup and L.-E. Swartz: Phys. Rev. Lett. 65 (1990) 452. T. Hayakawa and M. Morishima: Appl. Phys. Lett. 59 (1991) 3321. D. Leonard, M. Krishnamurthy, C. M. Reaves, S. P. Denbaars and P. M. Petroff: Appl. Phys.. Lett. 63 (1993) 3203. D. I. Lubyshev, P. P. Gonzalez-Borrero, E. Marega, Jr., E. Petitprez, La Scala, Jr. and P. Basmaji: Appl. Phys. Lett. 68 (1996) 205. D. I. Lubyshev, P. P. Gonzalez-Borrero, E. Marega, Jr., E. Petitprez and P. Basmaji: J. Vac. Sci. Technol. B 14 (1996) 2212. F. Y. Tsai and C. P. Lee: Jpn. J. Appl. Phys. 38 (1999) 558. H. H. Cheng, R. J. Nicholas, A. Priest, F. Y. Tsai, C. P. Lee, J. Sanchez-Dehesa: Physica B 256-258 (1998) 178. S. L. Tyan, F. Y. Tsai, C. P. Lee, P. A. Shields, and R. J. Nicholas: submitted to solid state comm... 450 C. 1/1119 1/1117 1/1115 1/1113. 931. 951. 971. W av e le n gt h (n m ) (c). 991. 1/1111 911. :11. 931. 9 51. 9 71. W av ele n gth ( nm ) ( d). 4. 99 1. :11.

(17) Fig. 1. In0.25Ga0.75As/GaAs QWs(all (100) samples and (111)B samples in (a),(b)) and QDs((111)B samples in (c), (d)) 4. 1/136. Voi gt con fi guration. C1. PL Intensit y (arb. units). 1/131. ∆E (meV). 3. 0.9 nm. C2 1/126. C3. Sample A Sample B Sample C Sample D. 2. 1 .2 n m. 1. 1 .5 n m. 0. 1/121. 0. C4 1/116. 1/111 911. 2 nm. C5. 2 .5 n m. C6. 3 nm. 931. 951. 971. 991. :11. :31. 2. 4. 6. 8. 10. Magne tic f ield (T ). : 51. W a v el en g t h ( n m ). Fig.2. QWs and QDs(pointed by arrows) of various thickness. Fig.3: The diamagnetic shifts of the PL lines, ∆E, for (111)B samples in Fig.1.. 5. 12. 14. 16.

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