The growth of an AlGaN/GaN modulation-doped heterostructure by NH3 source molecular beam epitaxy

Using NH3 cracked on the growing surface as the nitrogen precursor, an AlGaN/GaN modulation-doped (MD) heterostructure without a buffer layer was grown on a nitridated sapphire substrate in a home-made molecular beam epitaxy (MBE) system. Though the Al composition is as low as 0.036, as deduced from photoluminescence (PL) measurements, the AlGaN barrier layer can be an efficient carrier supplier for the formation of a two-dimensional electron gas (2DEG) at the heterointerface. The 2DEG characteristics are verified by the variable temperature Hall measurements down to 7 K. Using a parallel conduction model, we estimate the actual mobility of the 2DEG to be 1100 cm(2)/V s as the sheet carrier density to be 1.0 x 10(12) cm(-2). Our results show that the AlGaN/GaN system is very suitable for the fabrication of high electron mobility transistors (HEMTs). (C) 1998 Elsevier Science B.V. All rights reserved.

Photoluminescence investigation of GaInP/GaAs multiple quantum wells grown on (001) and (311) B GaAs surfaces by gas source molecular beam epitaxy

Photoluminescence (PL) investigation was carried out on GaInP/GaAs multiple quantum wells structures grown on (001) and (311) B surfaces of GaAs by gas source molecular beam epitaxy. Superlattice structures of GaAs/GaInP grown on (001) GaAs substrate were also studied in comparison. Deep-level luminescence was seen to dominate the PL spectra from the quantum wells and superlattice structures that were grown on (001) GaAs substrate. In contrast, superior optical properties were exhibited in the same structures grown on (311) B GaAs surfaces. The results suggested that GaAs/GaInP quantum well structures on (311) B oriented substrates could efficiently suppress the deep-level emissions, result in narrower PL peaks indicating smooth interfaces. (C) 1998 American Institute of Physics.

GaN epilayers grown at high growth rate using gas source molecular beam epitaxy method

High-quality GaN epilayers have been grown by gas source molecular beam epitaxy using ammonia as the nitrogen source. During the growth, the growth rate is up to 1.2 mu m/h and can be varied from 0.3 to 1.2 mu m. The unintentional n-type doping as low as 7x10(17) cm(-3) was obtained at room temperature. Low-temperature photoluminescence spectrum was dominated by near-edge emission without deep-level related luminescence, indicative of high-quality epilayers. (C) 1998 Elsevier Science B.V. All rights reserved.

Properties of GaN epilayers with various growth conditions grown by gas source molecular beam epitaxy

GaN epilayers on sapphire (0001) substrates were grown by the gas source molecular beam epitaxy (GSMBE) method using ammonia (NH,) gas as the nitrogen source. Properties of gallium nitride (GaN) epilayers grown under various growth conditions were investigated. The growth rate is up to 0.6 mu m/h in our experiments. Cathodoluminescence, photoluminescence and Hall measurements were used to characterize the films. It was shown that the growth parameters have a significant influence on the GaN properties. The yellow luminescence was enhanced at higher growth temperature. And a blue emission which maybe related to defects or impurity was observed. Although the emission at 3.31 eV can be suppressed by a low-temperature buffer layer, a high-quality GaN epilayer can be obtained without the buffer layer. (C) 1998 Elsevier Science B.V. All rights reserved.

Characterization of strain relaxation in As ion implanted Si1-xGex epilayers grown by gas source molecular beam epitaxy

Strain relaxation in the As ion implanted Si0.57Ge0.43 epilayers was studied by double-crystal x-ray diffractometry and transmission electron microscopy, and was compared to that in the nonimplanted Si0.57Ge0.43 epilayers. Experimental results show that after rapid thermal annealing (RTA) the x-ray linewidth of the As+-implanted Si0.57Ge0.43 epilayers is narrower than that of the nonimplanted epilayers, and than that of the partially relaxed as-grown samples, which is due primarily to low density of misfit dislocations in the As+-implanted SiGe epilayers. RTA at higher than 950 degrees C results in the formation of misfit dislocations for the nonimplanted structures, and of combinations of dislocations and precipitates (tentatively identified as GeAs) for the As+-implanted epilayers. The results mean that the strain relaxation mechanism of the As+-implanted Si1-xGex epilayers may be different from that of the nonimplanted Si1-xGex epilayers. (C) 1998 American Institute of Physics.

Influence of rapid thermal annealing on the optical properties of gallium nitride grown by gas-source molecular-beam epitaxy

Raman scattering, photoluminescence (PL), and nuclear reaction analysis (MA) have been employed to investigate the effects of rapid thermal annealing (RTA) on GaN films grown on sapphire (0001) substrates by gas-source molecular-beam epitaxy, The Raman spectra showed the presence of the E-2 (high) mode of GaN and shift of this mode from 572 to 568 cm(-1) caused by annealing. The results showed that RTA has a significant effect on the strain relaxation caused by the lattice and thermal expansion misfit between the GaN epilayer and the substrate. The PL peak exhibited a blueshift in its energy position and a decrease in the full width at half maximum after annealing, indicating an improvement in the optical quality of the film. Furthermore, a green luminescence appeared after annealing and increased in intensity with increasing annealing time. This effect was attributed to H concentration variation in the GaN film, which was measured by NRA. A high H concentration exists in as-grown GaN, which can neutralize the deep level, and the H-bonded complex dissociates during RTA, This leads to the appearance of a luminescent peak in the PL spectrum. (C) 1998 American Institute of Physics.

Structural identification of a cubic phase in hexagonal GaN films grown on sapphire by gas-source molecular beam epitaxy

The structural characteristics of gallium nitride (GaN) films grown on sapphire(0001) substrates by gas source molecular beam epitaxy (GSMBE) have been investigated using high-resolution synchrotron irradiation X-ray diffraction and cathodoluminescence with a variable energy electron beam. Besides the well-known GaN hexagonal structure, a small portion of cubic phase GaN was observed. The X-ray measurements provide an essential means for the structural identification of the GaN layers. Arising from the variable penetration depth of the electron beam in the cathodoluminescence measurements, it was found that the fraction of the GaN cubic-phase typically increased as the probing depth was increased. The results suggest that the GaN cubic phase is mostly located near the interface between the substrate and GaN layer due to the initial nucleation.

Widely Frequency-Tunable Optical Microwave Source Based on Amplified Feedback Laser

A monolithic integrated amplified feedback semiconductor laser is demonstrated as an optical microwave source. The optical microwave frequency is continuously tunable in the range of 19.87-26.3 GHz with extinction ratio above 6 dB, 3-dB linewidth about 3MHz.

High resistance AlGaAs/GaAs quantum cascade detectors grown by solid source molecular beam epitaxy operating above liquid nitrogen temperature

This work was supported by the National Research Projects of China (grant numbers are 60525406, 60736031, 60806018, 60906026, 2006CB604903, 2007AA03Z446 and 2009AA03Z403, 10990100, respectively). The authors would like to thank P Liang, Y Hu, H Sun, X L Zhang, B J Sun, H L Zhen and N Li for their help in processing and characterization.

measuring acoustic wave transit time in furnace based on active acoustic source signal

Accurate measurement of transit time for acoustic wave between two sensors installed on two sides of a furnace is a key to implementing the temperature field measurement technique based on acoustical method. A new method for measuring transit time of acoustic wave based on active acoustic source signal is proposed in this paper, which includes the followings: the time when the acoustic source signal arrives at the two sensors is measured first; then, the difference of two arriving time arguments is computed, thereby we get the transit time of the acoustic wave between two sensors installed on the two sides of the furnace. Avoiding the restriction on acoustic source signal and background noise, the new method can get the transit time of acoustic wave with higher precision and stronger ability of resisting noise interference.

Superluminal radio source PKS 0420-014

The very long baseline interferometry result of a superluminal radio source PKS 0420-014 at 5 GHz with Shanghai (China), Urumqi (China), Note (Italy), and HartRAO (South Africa) telescopes is presented. Proper motions of the relativistic jet components in the source are calculated. Based on the Self-Compton emission in a uniform spherical model, the beaming parameters of the source are estimated. The results show that PKS 0420-014 has a high Doppler factor of 9.3, a Lorentz factor of 6.5, and a small angle of 5.5 degrees to the line of sight.

HIGH-QUALITY INGAP AND INGAP/INALP MULTIPLE-QUANTUM-WELL GROWN BY GAS-SOURCE MOLECULAR-BEAM EPITAXY

Using gas-source molecular beam epitaxy, we have obtained high-quality GaInP and (AlGa)InP epilayers lattice-matched to (100) GaAs substrates. All grown layers exhibited mirror-like surfaces. For a 1.7 mum thick Ga0.5In0.5P film, the Hall electron mobility was 3400 and 30,000 CM2/V. s at 300 and 77 K, respectively. The luminescence wavelength of (AlxGa1-x)InP samples ranged from 680 nm (for GaInP) to 590 nm (for AlInP) at room temperature, and from 644 to 513 nm at 77 K. The multiple quantum well (MQW) structure with well width of 40 angstrom showed strong luminescence intensity with wavelength of 647 nm (300 K) or 622 nm (80 K). The satellite peaks can be detected in double-crystal X-ray (DCXR) diffraction measurements of the MQW samples, which indicates the perfect structural periodicity.

Some properties of gallium nitride films grown on (0001) oriented sapphire substrates by gas source molecular beam epitaxy

The Raman and photoreflectivity spectra of gallium nitride (GaN) films grown on (0001) oriented sapphire substrates by gas source molecular beam epitaxy (GSMBE) have been investigated. The Raman spectra showed the presence of the E-2(high) mode and a shift in the wavenumber of this mode with respect to the GaN epilayer thickness. The Raman scattering results suggest the presence of stress due to lattice and thermal expansion misfit in the films, and also indicate that the buffer layer play an important role in the deposition of high quality GaN layers. The residual stress changes from tensile to compressive as the epilayer thickness increases. Samples subjected to anneal cycles showed an increase in the mobility due probably to stress relaxation as suggested by an observed shift in the E-2(high) mode in the Raman spectra after annealing.