Ma Ziguang

Stress Control in GaN Grown on 6H-SiC by Metalorganic Chemical Vapor Deposition

The strain in GaN epitaxial layers grown on 6H-SiC substrates with an AlN buffer by metalorganic chemical vapor deposition is investigated. It is found that the insertion of a graded AlGaN layer between the GaN layer and the AlN buffer can change the signs of strain. A compressive strain in an overgrown thick (2 μm) GaN layer is obtained. High-resolution x-ray diffraction, Raman spectroscopy and photoluminescence measurements are used to determine the strain state in the GaN layers. The mechanism of stress control by inserting graded AlGaN in subsequent GaN layers is discussed briefly.

The Influence of Graded AlGaN Buffer Thickness for Crack-Free GaN on Si(111) Substrates by using MOCVD

GaN films with different thicknesses of Al composition graded AlGaN buffer are grown on substrates of Si(111) by metal-organic chemical vapor deposition (MOCVD). The thicknesses of graded AlGaN buffer are fixed at 200 nm, 300 nm, and 450 nm, respectively. Optical microscopy, atomic force microscopy, x-ray diffraction, and Raman spectroscopy are employed to characterize these samples. We find that the thickness of the graded AlGaN buffer layer plays a key role on the following growth of GaN films. The optimized thickness of the graded AlGaN buffer layer is 300 nm. Under such conditions, the GaN epitaxial film is crack-free, and its dislocation density is the lowest.

1.0 eV GaAs based InAs quantum dot solar cells

There are great improvements in III-V semiconductor solar cells including GaAs single junction solar cell and tandem solar cells in last decades. The common commercially used triple-junction solar cell mostly utilize a Ge bottom cell with an (In)GaAs and InGaP middle and top cell respectively. The (In)GaAs middle cell absorbs limited solar spectrum and show minimum short-circuit current in multiple junction solar cell, which determine the overall current of the cell. Theoretically, higher energy conversion efficiency will be obtained when introducea subcell with 1.0 eV band gap in series to the InGaP/GaAs/Ge tandem structures. Lattice-matched GaInNAs and lattice-mismatched In0.3Ga0.7As are two common choices for achieving 1.0 eV band gap. However, the addition of N to GaAs has detrimental effects on the material quality, limiting its utility to approaches that required photocurrent. And the lattice mismatch makes thick In0.3Ga0.7As heterogeneous epitaxial difficult. The incorporation of low band gap nanoscale materials, such as quantum well and quantum dots, into current limiting junction of a multijunction solar cell can tuning the effective bandgap and enhance the low-energy photon absorption, thereby increasing the short-circuit current. The samples are grown by solid-source molecular beam epitaxy (V80H) on p-GaAs (001) substrates using an As4 source. Sample A is a standard GaAs solar cell with PIN structure. Sample B contains 10 period 8 nm In0.15Ga0.85As QW and sample C contains 10 period InAs dots-in-well structure. The intrinsic region of these samples was at the same length of 630 nm to obtain the same build-in electric field. Besides the instinct region, the three samples are designed the same. The ohmic contact layer’s doping concentration is N D/ N A=3×1018 cm−3 and its thickness is 300 nm. The emitter doping concentration is N D=5×1017 cm−3 and its thickness is 150 nm; The base doping concentration is N A=5×1017 cm−3 and its thickness is 300 nm. The As4 pressure and the InAs QDs growth rate are 1×10−5 Torr and 0.1 ML/s, respectively. The XRD pattern of sample B (QWSC) shows clearly defined and intense satellite peaks indicating steep interface and periodicity quantum well. However, the XRD pattern of sample C (the InAs dots in well solar cell) shows blurring diffraction peaks which means increased interface roughness. Uniform QD structure with density of 2×1010 cm−2 can be observed in SEM. From the STEM, we can see well-defined unrelated pyramidal quantum dots separated by a nominal 50 nm GaAs spacing. From the contrast, the pyramid InAs QDs are estimated to have the lateral dimension of 27 nm and the height of 12 nm. The PC response of sample A rise at 600 nm and fall off at 870 nm, which is corresponding to the GaAs bandgap according to the PL peak position; the PC response of QWSC (sample B) rise at 600 nm and fall off at 990 nm, which is corresponding to the In0.15Ga0.85As bandgap in PL spectrum. The incorporation of In0.15Ga0.85As quantum wells in GaAs standard solar extend the absorption of solar spectrum from 900 nm to 1000 nm. The wavelength region from 700 nm to 900 nm in PC of QWSC is weaker than the GaAs standard solar cell. The insertion of InAs QDs into QWSC can extend the infrared photon absorption to the wavelength of 1300 nm. The insertion of InAs quantum dots into In0.15Ga0.85As/GaAs quantum well solar cell extend the absorption of infrared light and the J sc reaches 13.68 mA/cm2, giving a 37.8% increase in short-circuit current over the baseline solar cell current. Because the voltage of multiple junction solar cell is the sum of the sub cell’s voltage. The voltage decreasement with the insertion of InAs quantum dots do not influence the overall voltage of multiple junction solar cell. In summary, the incorporation of quantum well and quantum dots in GaAs solar cell result in extended solar spectrum absorption and contribute to the sub-band gap current gains for GaAs solar cell. Especially, InAs dots-in-well solar cell can extend the absorption spectrum to the wavelength of 1300 nm, and increase by 37.8% in short-circuit current, which shows great potential in mitigating current mismatch of multiple junction solar cell. Furthermore, designing new multiple junction solar cell becomes possible.

Luminescent Performances of Green InGaN/GaN MQW LED Employing Superlattices Strain Adjusting Structures

Green InGaN/GaN multiple quantum well(MQW) LEDs employing InGaN/GaN superlattice(SL) structure were studied.The distribution of indium within the MQWs is changed by inserting the InGaN/GaN SL.Meanwhile,the average indium content of MQW does not change.Two InGaN-related peaks that were clearly found in the electroluminescence(EL) and photoluminescence(PL) spectrum,which are assigned to In-rich quantum dots(QD) and the InGaN matrix,respectively.It is suggested that the carrier drifts from the InGaN matrix to the In-rich QD.It could be concluded that employing SL structures is an effective way to adjust the wavelength of InGaN/GaN MQW without introducing new defects in the MQWs.