• Korean Journal of Materials Research
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• v.16 no.3
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• pp.183-187
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• 2006

We present the effects of rapid thermal annealing (RTA) temperature on the structural and optical properties of self-assembled InAs quantum dot (QD) structures grown on GaAs substrates by molecular beam epitaxy (MBE). The photoluminescence (PL) measurements are performed in a closed-cycle refrigerator as a function of temperature for the unannealed and annealed samples. RTA at higher temperature results in the increase in island size, the corresponding decrease in the density of islands, and the redshift in the PL emission from the islands. The temperature dependence of the PL peak energy for the InAs QDs is well expressed by the Varshni equation. The thermal quenching activation energies for the samples unannealed and annealed at $600^{\circ}C$ are found to be $25{\pm}5meV$ and $47{\pm}5$ meV, respectively.

• Journal of the Korean Vacuum Society
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• v.22 no.1
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• pp.37-44
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• 2013

In order to investigate an influence of carrier trap by quantum dots (QDs) on the solar parameters, in this study, the $p^+-QD-n/n^+$ solar cells with InAs/GaAs QD active layers are fabricated, and their characteristics are investigated and compared with those of a GaAs matrix solar cell (MSC). Two different types of QD structures, the Stranski-Krastanow (SK) QD and the quasi-monolayer (QML) QD, have been introduced for the QD solar cells, and the parameters (open-circuit voltage ($V_{OC}$), short-cirucuit current ($I_{SC}$), fill factor (FF), conversion efficiency (CE)) are determined from the current-voltage characteristic curves under a standard solar illumination (AM1.5). In SK-QSC, while FF of 80.0% is similar to that of MSC (80.3%), $V_{OC}$ and $J_{SC}$ are reduced by 0.03 V and $2.6mA/cm^2$, respectively. CE is lowered by 2.6% as results of reduced $V_{OC}$ and $J_{SC}$, which is due to a carrier trap into QDs. Though another alternative structure of QML-QD to be expected to relieve the carrier trap have been firstly tried for QSC in this study, it shows negative results contrary to our expectations.

• Korean Journal of Materials Research
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• v.29 no.9
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• pp.542-546
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• 2019

We investigate the characteristics of self-assembled quantum dot infrared photodetectors(QDIPs) based on doping level. Two kinds of QDIP samples are prepared using molecular beam epitaxy : $n^+-i(QD)-n^+$ QDIP with undoped quantum dot(QD) active region and $n^+-n^-(QD)-n^+$ QDIP containing Si direct doped QDs. InAs QDIPs were grown on semi-insulating GaAs (100) wafers by molecular-beam epitaxy. Both top and bottom contact GaAs layer are Si doped at $2{\times}10^{18}/cm^3$. The QD layers are grown by two-monolayer of InAs deposition and capped by InGaAs layer. For the $n^+-n^-(QD)-n^+$ structure, Si dopant is directly doped in InAs QD at $2{\times}10^{17}/cm^3$. Undoped and doped QDIPs show a photoresponse peak at about $8.3{\mu}m$, ranging from $6{\sim}10{\mu}m$ at 10 K. The intensity of the doped QDIP photoresponse is higher than that of the undoped QDIP on same temperature. Undoped QDIP yields a photoresponse of up to 50 K, whereas doped QDIP has a response of up to 30 K only. This result suggests that the doping level of QDs should be appropriately determined by compromising between photoresponsivity and operating temperature.

• Proceedings of the Korean Vacuum Society Conference
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• 2011.08a
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• pp.134-135
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• 2011

양자점은 전자와 양공을 3차원으로 속박 시키므로 기존의 bulk나 양자우물보다 양자점을 이용한 레이저 다이오드의 경우 낮은 문턱 전류, 높은 미분이득 및 온도 안전성의 장점이 있을 거라 기대되고 있다. 그러나, 양자점은 낮은 areal coverage 때문에 높은 속박효율을 얻지 못하고 있다. 이러한 양자점의 문제점을 해결하기 위해 양자점을 양자우물 안에 성장시켜 운반자들의 포획을 향상시키는 방법들이 연구되고 있다. 양자우물 안에 양자점을 넣으면 양자우물이 운반자들의 포획을 증가 시키고, 열적 방출도 억제하여 온도 안정성이 향상 되는 것으로 알려져 있다. 광통신 대역의 1.3 ${\mu}m$ 경우, GaAs계를 이용하여 InAs 양자점을 strained InGaAs 박막을 우물층으로 한 dot-in-a-well 구조의 연구는 몇몇 보고된 바 있다. 그러나 InP계를 사용하는 1.55 ${\mu}m$ 대역에서 dot-in-a-well구조의 연구는 아직 미미하다. 본 연구에서는 유기 금속 화학 증착법(metal organic chemical vapor deposition)을 이용하여 InP 기판 위에 InAs 양자점을 자발성장법으로 성장하였으며 dot-in-a-well 구조에서 우물층으로 1.35 ${\mu}m$ 파장의 $In_{0.69}Ga_{0.31}As_{0.67}P_{0.33}$ (1.35Q)를, 장벽층으로는 1.1 ${\mu}m$ 파장의 $In_{0.85}Ga_{0.15}As_{0.32}P_{0.68}$(1.1Q)를 사용하였다. 양자우물층과 장벽층은 모두 InP 기판과 격자가 일치하는 조건으로 성장하였다. III족 원료로는 trimethylindium (TMI)와 trimethylgalium (TMGa)을 사용하였으며 V족 원료 가스로는 $PH_3$ 100%, $AsH_3$ 100%를, carrier gas로는 $H_2$를 사용하였다. InP buffer층의 성장 온도는 640$^{\circ}C$이며 양자점 성장 온도는 520$^{\circ}C$이다. 양자점 형성은 원자력간 현미경(Atomic force microscopy)를 이용하여 확인하였으며, 박막의 결정성은 쌍결정 회절분석(Double crystal x-ray deffractometry)를 이용하여 확인하였다. 확인된 성장 조건을 이용하여 양자점 시료를 성장하였으며 광여기분광법(Photoluminescence)을 이용하여 광특성을 분석하였다. Fig. 1은 dot in a barrier 와 dot-in-a-well 시료의 성장구조이다. Fig. 1(a)는 일반적인 dot-in-a-barrier 구조로 InP buffer층을 성장하고 1.1Q를 100 nm 성장한 후 양자점을 성장하였다. 그 후 1.1Q 100 nm와 InP 100 nm로 capping하였다. Fig. 1(b)는 dot-in-a-well 구조로 InP buffer층을 성장하고 1.1Q를 100 nm 성장 후 1.35Q 우물층을 4 nm 성장하였다. 그 위에 InAs 양자점을 성장하였다. 그 후에 1.35Q 우물층을 4 nm 성장하고 1.1Q 100 nm와 InP 100 nm로 capping하였다. Fig. 2는 dot-in-a-barrier 시료와 dot-in-a-well 시료의 상온 PL data이다. Dot-in-a-barrier 시료의 PL 파장은 1544 nm이며 반치폭은 79.70 meV이다. Dot-in-a-well 시료의 파장은 1546 nm이며 반치폭은 70.80 meV이다. 두 시료의 PL 파장 변화는 없으며, 반치폭은 dot-in-a-well 시료가 8.9 meV 감소하였다. Dot-in-a-well 시료의 PL peak 강도는 57% 증가하였으며 적분강도(integration intensity)는 45%가 증가하였다. PL 데이터에서 높은 에너지의 반치폭 변화는 없으며 낮은 에너지의 반치폭은 8 meV 감소하였다. 적분강도 증가에서 dot-in-a-well 구조가 dot-in-a-barrier 구조보다 전자-양공의 재결합이 증가한다는 것을 알 수 있으며, 반치폭 변화로부터 특히 높은 에너지를 갖는 작은 양자점에서의 재결합이 증가 된 것을 알 수 있다. 이는 양자우물이 장벽보다 전자-양공의 구속력을 증가시키기 때문에 양자점에 전자와 양공의 공급을 증가시키기 때문이다. 따라서 낮은 에너지를 가지는 양자점을 모두 채우고 높은 에너지를 가지는 양자점까지 채우게 되므로, 높은 에너지를 가지는 양자점에서의 전자-양공 재결합이 증가되었기 때문이다. 뿐만 아니라 파장 변화 없이 PL peak 강도와 적분강도가 증가하고 낮은 에너지 쪽의 반치폭이 감소한 것으로부터 에너지가 낮은 양자점보다는 에너지가 높은 양자점에서의 전자-양공 재결합율이 급증하였음을 알 수 있다. 우리는 이와 같은 연구에서 InP계를 이용해 1.55 ${\mu}m$에서도 dot in a well구조를 성장 하여 더 좋은 특성을 낼 수 있으며 앞으로 많은 연구가 필요할 것이라 생각한다.

• Bulletin of the Korean Chemical Society
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• v.30 no.4
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• pp.857-862
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• 2009

We model lattice-mismatched group III-V semiconductor $In_{x}Ga_{1-x}$ alloys with the three-parameter anharmonic Kirkwood-Keating potential, which includes realistic distortion effect by introducing anharmonicity. Although the potential parameters were determined based on optical properties of the binary parent alloys InAs and GaAs, simulated dielectric functions, reflectance, and Raman spectra of alloys agree excellently with experimental data for any arbitrary atomic composition. For a wide range of atomic composition, InAs- and GaAs-bond retain their respective properties of binary parent crystals despite lattice and charge mismatch. It implies that use of the anharmonic Kirkwood-Keating potential may provide an optimal model system to investigate diverse and unique optical properties of quantum dot heterostructures by circumventing potential parameter searches for particular local structures.

• Journal of the Korean Vacuum Society
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• v.18 no.5
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• pp.352-357
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• 2009

The InAs multi-quantum dots (MQDs) solar cell and InGaAs multi-quantum wells (MQWs) solar cell to cover 1.1 eV and 1.3 eV were designed by 1D poisson, respectively. The MQDs and MQWs of 5, 10, 15 layers were grown by molecular beam epitaxy. The photo luminescence results showed that the 5 period stacked MQDs have the highest intensity at around 1.1 eV with 57.6 meV full width at half maximum (FWHM). Also we can observe 10 period stacked MQWs peak position which has highest intensity at 1.31 eV with 12.37 meV FWHM. The density and size of QDs were observed by reflection high energy electron diffraction pattern and atomic force microscope. Futhermore, AlGaAs/GaAs sandwiched tunnel junctions were modified according to the width of GaAs layer on p-type GaAs substrates. The structures with GaAs width of 30 nm and 50 nm have backward diode characteristics. In contrast, tunnel diode characteristics were observed in the 20 nm of that of sample.

• Proceedings of the Korean Vacuum Society Conference
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• 2010.08a
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• pp.234-234
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• 2010

The PR transitions in asymmetric dot-in-double-quantum-well (DdWELL) photodetector is identified by bias-dependent spectral behaviors. Discrete n-i-n infrared photodetectors were fabricated on a 30-period asymmetric InAs-QD/[InGaAs/GaAs]/AlGaAs DdWELL wafer that was prepared by MBE technique. A 2.0-monolayer (ML) InAs QD ensemble was embedded in upper combined well of InGaAs/GaAs and each stack is separated by a 50-nm AlGaAs barrier. Each pixel has circular aperture of 300 um in diameter, and the mesa cell ($410{\times}410\;{\mu}m^2$) was defined by shallow etching. PR measurements were performed in the spectral range of $3{\sim}13\;{\mu}m$ (~ 100-400 meV) by using a Fourier-transform infrared (FTIR) spectrometer and a low-noise preamplifier. The asymmetric photodetector exhibits unique transition behaviors that near-/far-infrared (NIR/FIR) photoresponse (PR) bands are blue/red shifted by the electric field, contrasted to mid-infrared (MIR) with no dependence. In addition, the MIR-FIR dual-band spectra change into single-band feature by the polarity. A four-level energy band model is proposed for the transition scheme, and the field dependence of FIR bands numerically calculated by a simplified DdWELL structure is in good agreement with that of the PR spectra. The wavelength shift by the field strength and the spectral change by the polarity are discussed on the basis of four-level transition.

• Journal of the Korean Vacuum Society
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• v.10 no.3
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• pp.374-379
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• 2001

We have investigated the annealing effects on the optical properties of InAs quantum dots(QDs) capped with InGaAs(sample QDl), where InGaAs layer was deposited by opening Gallium, Arsenic, Indium and Arsenic shutters alternately with 3 periods, grown by molecular beam epitaxy. The emission wavelength of the sample of InAs QDs capped by GaAs barriers was observed to be blue-shifted as the annealing temperature was increased. On the other hand, the photoluminescence(PL) peak position of sample QD1 was observed to be red-shifted at the annealing temperature of up to $600^{\circ}C$ and, then, it was found to be blue-shifted at temperatures ranging from 700 to $800^{\circ}C$. The full width at half maximum values of sample QD1 subjected to annealing treatments show different behavior compared to typical InAs quantum dot structures.

• 한국전자현미경학회:학술대회논문집
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• 2005.05a
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• pp.66-69
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• 2005

• Proceedings of the Korean Vacuum Society Conference
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• 2006.02a
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• pp.203-203
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• 2006