Buffer layer Silicon substrate
8.3 Experiment result
After the growth, the sample A shows a non-mirror surface, but the sample B and C shows a better mirror surface. Therefore, the surface morphology of sample B and C is much smoother than that of sample A. Fig. 8.2 shows the surface AFM image of these three samples. All of the AFM image size is 10μm×10μm with 30nm scale bar. The surface roughness of sample A is the worst, and the other two samples are much similar. The surface roughness is quantified by the atomic force microscope (AFM). The root mean square surface roughness of sample B and C are 4.27nm and 3.88nm with the same scanning parameter. The sample C showes the best surface flatness quality. The flatter surface is better for the structure growth.
A B C
Figure 8.2: The surface AFM image of these three samples. The image size is 10μm×10μm with the 30nm scale bar.
Fig. 8.3 shows the In0.2Ga0.8Sb/GaSb QW low temperature (20K) photoluminescence (PL) response of these three samples. The sample C shows the largest response intensity and the smallest full width at half maximum (FWHM). The FWHM of sample C is 11.2meV. The peak emission energy of the In0.2Ga0.8Sb QW varies from 0.71eV to 0.73eV. The peak energy variation might be due to the source beam flux variation. With simple estimation, 2% material composition difference on In0.2Ga0.8Sb QW results in 11meV energy level change.
0.69 0.70 0.71 0.72 0.73 0.74 0.75
0 1 2 3 4 5 6 7
PL Response (a. u.)
Energy (eV)
Sample A Sample B Sample C
20K
Figure 8.3: The 20K PL response from the 8nm In0.2Ga0.8Sb/GaSb QW. The FWHM of sample C is 11.2meV.
Fig. 8.4 shows the high resolution X-ray diffraction (XRD) rocking curve from GaSb (004) orientation of the three samples. The scatter point is the measurement data and continuous curve is the gaussian fitting curve. These three samples were measured at the same condition and the same X-ray source intensity. The sample C shows the higher intensity and smaller FWHM. The FWHM of sample C is 490 arcsec,
In our study, three analysis techniques (AFM, PL and XRD) show the GaSb bulk and In0.2Ga0.8Sb/GaSb QW quality of sample C is the best and sample A is worst. The AlSb buffer layer is necessary in the crystal growth of GaSb on Silicon substrate.
Moreover, the GaSb/AlSb superlattice buffer layer is the best choice for the growth.
-1000 -500 0 500 1000
0 500 1000 1500 2000 2500
A B C
Intensi ty (a.u.)
ω (arcsecond)
Figure 8.4: High resolution XRD (004) rocking curve of GaSb on Silicon. The FWHM of the sample C is 490 arcsec.
8.4 Discussion
The lattice constant of GaSb and silicon is 6.10 Å and 5.43 Å, and the lattice mismatch is about 12%. When GaSb deposited directly on silicon substrate, it would generate many dislocations at the interface. The non-mirror surface of sample A suggests that the GaSb does not block dislocations propagation efficiently. When 100nm AlSb buffer layer was inserted at GaSb and silicon interface in sample B, the GaSb crystal shows a mirror surface. The surface AFM image of sample B is smoother than that of sample A and the PL and XRD study also shows the better result.
The In0.2Ga0.8Sb/GaSb QW PL intensity of sample B is two times larger than that of sample A. Therefore, the AlSb buffer layer plays an important role in the GaSb and silicon heterojunction growth. The lattice constant of AlSb is 6.13 Å, and the lattice mismatch to silicon is about 13%. When AlSb deposited on silicon, many dislocations generate. Observing the RHEED pattern variation during sample growth, the reconstruction pattern is spotty during the first few monolayers. The few monolayers AlSb form the QDs, which is the nucleation process, on the Silicon surface [37, 71-73]. When more AlSb deposited on the substrate, the reconstruction pattern changes from spotty to streaks. The streaky reconstruction pattern indicates that the AlSb QDs coalesce to a bulk material. This nucleation and coalescence process generates an undulation surface, which accommodates the AlSb and silicon heterointerface strain energy. These processes are the strain relief mechanism.
In sample C, the QW PL intensity shows three times larger than sample B, and the FWHM is also better than sample B. Also, the intensity and FWHM of the XRD measurement data shows the best result in the three samples. Thus, the ten periods superlattice GaSb (10nm)/AlSb (10nm) buffer layer is the best buffer layer structure to prevent the dislocation propagation in our study. The lattice constant mismatch between AlSb and GaSb is only 0.5% and thus the heterointerface is nearly strain free.
The superlattice structure is able to merge dislocations and stop dislocation propagation. The first 10nm AlSb layer also plays as the nucleation and coalescence process. The 10nm AlSb layer is about 30 monolayers that are thick enough to coalesce the AlSb QDs and form the undulation surface. During the 10nm AlSb growth, the reconstruction pattern changes from spotty to streaky pattern. Fig. 8.5 shows the GaSb/AlSb superlattice high resolution transmission electron microscope (HR-TEM) image of sample C. In the area 1 and 2, the dislocations merge at the GaSb/AlSb interface. And, in the area 3, the dislocation stops at the interface.
Figure 8.5: Cross-section HR-TEM image of the GaSb/AlSb superlattice on silicon substrate in sample C. In area 1, 2 and 3 the dislocations merge or stop at the superlattice interface.
8.4 Summary
The heterojunction growth of GaSb on silicon (001) substrate with different buffer layer has been studied. When the GaSb deposited directly on the silicon surface, the GaSb crystal surface shows a non-mirror surface. It is necessary to use the AlSb as the buffer layer material to enhance the crystal quality. This AlSb nucleation and coalescence processes accommodate the heterojunction strain energy and result in better crystal quality. Furthermore, the GaSb/AlSb superlattice interface would merge the dislocations and block the dislocation propagation. This buffer layer structure results in better GaSb crystal quality.
Chapter 9 Conclusion
In this dissertation, the study of the quantum dot infrared photodetectors (QDIPs) and GaSb material are presented at the former chapters. The primary conclusion is summarized as follows:
The first part: quantum dot infrared photodetectors (QDIPs):
The responsivity temperature dependence behavior of InAs/GaAs QDIPs has been investigated. From the measurement, we found the dramatic change of the current gain with temperature dominates the behavior of the responsivity. The increasing dark current with the temperature injects more carriers into the QDs. The repulsive potential of the extra carriers suppresses the capture process and enhances the current gain. The average extra carrier numbers calculated from the capture probability qualitatively explained the behavior of the quantum efficiency. From this concept, QDIPs with smaller QD and higher density is predicted to have better temperature stability and also maintain a higher current gain.
The vertically coupled QDIPs have been investigated. With the mini-bands in the coupled QDs, the photoresponse spectrum, quantum efficiency, and the roll-off frequency for QDIPs have been improved. The photoresponse spectrum shows a smaller fractional spectral width of 6% (Δλ/λp), which increased the quantum efficiency. The mini-bands also enhanced the capture probability and the roll-off frequency dramatically. The device provided a possible solution to enhance the quantum efficiency and roll-off frequency at the same time. More works to improve the vertical alignment of the QDs will further improve the performance of the
vertically coupled QDIPs.
The QDIPs with a thin wide band gap material Al0.2Ga0.8As layer near QDs have been studied. The board photocurrent spectra of the simple InAs/GaAs QDIPs are found to be composed of two transitions. With the insertion of the Al0.2Ga0.8As layer, the response spectra could be separated into two peaks. One of the peaks is fixed at 6μm, and the other shifts to higher energy and the intensity becomes weaker as the decrease of the Al0.2Ga0.8As layer distance. The much narrow photocurrent spectrum width is obtained, and the fractional spectrum width is reduced from 25% to 10% as the Al0.2Ga0.8As layer is 5nm from the QD layer. Because one of the transitions is suppressed and the carrier contributes to the other transition, the quantum efficiency increases. Also, the thin Al0.2Ga0.8As layer reduces the device dark current, and then the detectivity is enhanced for 5 times.
The second parts: GaSb/GaAs QDs and GaSb growth on silicon substrate:
The growth conditions of the GaSb quantum structure in GaAs (001) matrix have been systematically investigated, including the film thickness, substrate temperature and V/III flux ratio. And, the photoluminescence results in different excitation power and temperature have also been studied. Due to the spatially separated electron and hole, the carrier life time of the type-II GaSb/GaAs QDs is much longer than the type-I InAs/GaAs QDs. This phenomenon would cause the PL signal blue shift with excitation power density and the QD PL signal blue shift with temperature.
Furthermore, we observed distinct PL peaks from monolayer GaSb layers in GaAs.
Such peaks have been identified to be originated from optical transitions in 1-, 2- and 3-ML GaSb/GaAs type-II QWs. A range of valence band offset for the GaSb/GaAs heterojunction from 0.4 to 0.6 eV is suggested according to the fitting of the measured data to our eight-band k•p calculation.
The heterojunction growth of GaSb on silicon (001) substrate with different buffer layer has been studied. When the GaSb deposited directly on the silicon surface, the GaSb crystal surface shows a non-mirror surface. It is necessary to use the AlSb as the buffer layer material to enhance the crystal quality. This AlSb nucleation and coalescence processes accommodate the heterojunction strain energy and result in better crystal quality. Furthermore, the GaSb/AlSb superlattice interface would merge the dislocations and block the dislocation propagation. This buffer layer structure results in better GaSb crystal quality.
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Vita
Name: Ming-Cheng, Lo (羅 明 城 ) Sex: Male
Date of Birth: October 8, 1980
Place of birth: Taipei, Taiwan, R.O.C.
Education:
National Chiao Tung University Ph. D September, 2003- Institute of Electronics Engineering
National Chiao Tung University M. S. September, 2002~June, 2003 Institute of Electronics Engineering
National Chiao Tung University B. S. September, 1998~June, 2002 Institute of Electronics Engineering
Title of the Ph. D Dissertation:
Studies of Quantum Dot Infrared Photodetectors and GaSb Material