X. M. Fang

IV–VI compound midinfrared high-reflectivity mirrors and vertical-cavity surface-emitting lasers grown by molecular-beam epitaxy

Midinfrared broadband high-reflectivity Pb1−xSrxSe/BaF2 distributed Bragg reflectors and vertical-cavity surface-emitting lasers (VCSELs) with PbSe as the active material were grown by molecular-beam epitaxy. Because of an extremely high index contrast, mirrors with only three quarter-wave layer pairs had reflectivities exceeding 99%. For pulsed optical pumping, a lead salt VCSEL emitting at the cavity wavelength of 4.5–4.6 μm operated nearly to room temperature (289 K).

Above-room-temperature continuous-wave mid-infrared photoluminescence from PbSe/PbSrSe quantum wells

Strong photoluminescence between 3 and 4 μm was observed at temperatures as high as 55 °C from PbSe/PbSrSe multiple-quantum-well structures grown on BaF2 (111) substrates by molecular-beam epitaxy. Fabry–Perot interference fringes dominated the spectra, indicating that the luminescence was primarily due to stimulated emission processes. Peak emission energies were determined by fitting Gaussian functions to the spectra, and they showed that emission energies at 25 °C decreased from 402 to 312 meV as quantum-well thickness increased from 40 to 200 A. The temperature tuning coefficient was also observed to decrease from 0.400 meV/K for a 200 A multiple-quantum-well sample to 0.313 meV/K for a 40 A multiple-quantum-well sample.

Molecular beam epitaxy of InSb on Si substrates using fluoride buffer layers

The molecular beam epitaxy of InSb/Si structures was accomplished using group IIa fluoride buffer layers. InSb growth was initiated by opening the In and Sb shutters simultaneously at substrate temperatures between 300 °C and 400 °C, producing In-terminated InSb(111)-A surfaces on CaF2/Si(111) substrates. Reflection high-energy electron diffraction, electron channeling, and high resolution x-ray diffraction measurements indicated that the InSb layers were of good crystalline quality. Electron mobilities at room temperature were as high as 65 000 cm2/V s for an 8-μm-thick InSb layer grown on CaF2/Si(111). On CaF2/Si(001) substrates, the InSb layers grew in the (111) orientation with two domains 90° apart. These InSb layers and ones grown on BaF2/CaF2/Si(111) substrates exhibited inferior electrical and structural properties compared to structures grown on CaF2/Si(111) substrates.

Molecular beam epitaxy growth of PbSe on BaF2-coated Si(111) and observation of the PbSe growth interface

Epitaxial growth of PbSe/BaF2/CaF2 heterostructures was carried out by molecular beam epitaxy (MBE) on Si(111) wafers. Successful transfer of 3-μm-thick PbSe epilayers was accomplished by bonding the MBE-grown samples face down to polished copper plates followed by the removal of the silicon substrate by dissolving the BaF2 buffer layer in water. High-resolution x-ray diffraction measurements demonstrated that the PbSe epilayer maintained high-crystalline quality after transfer. In addition, optical Nomarski characterization of the exposed growth interface showed sets of parallel straight step lines consistent with glide of dislocations in the primary {100}〈110〉 glide system. Such features are evidence of the large thermal expansion mismatch strain that occurred in these layers.

Growth studies of CaF2 and BaF2CaF2 on (100) silicon using RHEED and SEM

Abstract Molecular beam epitaxial growth of CaF 2 and BaF 2 CaF 2 stacked layers on (100) silicon substrates was studied using reflection high-energy electron diffraction and scanning electron microscopy. Both in-situ annealing and a two-stage growth method were applied to improve the crystallinity and surface morphology of the epitaxial layers. Smooth, facet-free, (100) -oriented CaF 2 layers on (100) silicon were obtained using both methods. BaF 2 layers grown on smooth CaF 2 -coated (100) silicon surfaces in the range 580–800 °C, however, exhibited mixed (111) and (100) orientations, whereas epitaxial growth of (100)-oriented BaF 2 was achieved on rough CaF 2 surfaces. It is believed that exposed {111} facets on rough CaF 2 layers provide favorable low-energy surfaces for nucleation of epitaxial BaF 2 . Smooth BaF 2 layers were obtained using a two-stage growth method where an initial BaF 2 layer, 1 000–3 000 A thick, was grown at 580 °C on a 300 A thick CaF 2 layer, annealed at 820 °C for a few minutes, and followed by 1 000 A of BaF 2 grown at 700–800 °C. Annealing the initial BaF 2 layer above 870 °C resulted in mixed (111) and (100) orientations, thus imposing an upper limit on this processing step.

Raman Scattering Study of PbSe Grown on (111) BaF2 Substrate

PbSe films were grown on (111)-oriented BaF2 substrates by using molecular beam epitaxy. High resolution x-ray diffraction characterization showed good crystalline quality of PbSe films. Both longitudinal optical phonon at 135 cm-1 and transverse optical phonon at 47.6 cm-1 were observed by Raman scattering measurements. The Raman tensor calculation demonstrates that both transverse-optical and longitudinal-optical (LO) phonons in PbSe crystal are Raman active on (111)-oriented surface. Furthermore, 2LO phonon at about 270 cm-1 and polaron at about 800 cm-1 in PbSe, were also observed. The observed Raman frequencies are in good agreement with theoretical calculations using point ion model.

MBE growth of PbSe/CaF2/Si(1 1 1) heterostructures

Successful epitaxial growth of PbSe/CaF 2 /Si(1 1 1) heterostructures using just one growth chamber of a molecular beam epitaxy system is described. Sharp (1 x 1) streaks in the RHEED pattern confirm the high structural quality of PbSe, and SEM and Nomarski microscopy reveal smooth PbSe surfaces. The PbSe epilayers show mobilities as high as 23 200 cm 2 V -1 s -1 and electron concentrations of 1-2 x 10 17 cm -3 at 77 K. In addition, observation of RHEED intensity oscillations confirm layer-by-layer growth modes for both CaF 2 and PbSe. The PbSe/CaF 2 interface has also been characterized by using X-ray photoelectron spectroscopy. All the Ca 2p, F 1s, Pb 4f and Se 3d peaks show chemical shifts as PbSe coverage on CaF 2 increases. Results show that the PbSe/CaF 2 interface consists of a thin transition layer dominated by Pb-F and Ca-Se bonds.

Growth and characterization of PbSe and Pb1−xSnxSe on Si (100)

PbSe and Pb1−xSnxSe layers, with thicknesses ranging from 1 to 5 μm, were grown by liquid phase epitaxy on Si (100) substrates using PbSe/BaF2/CaF2 buffer layers grown by molecular beam epitaxy. Optical Nomarski characterization revealed excellent surface morphologies and good growth solution wipeoffs. Although most PbSe layers were free of cracks over the entire 8×8 mm2 substrate area, ternary Pb1−xSnxSe layers exhibited varying crack densities ranging from zero in the center of samples to over 30 cracks/cm at the edges. High resolution x-ray diffraction (HRXRD) measurements of crack-free PbSe layers showed a residual in-plane tensile strain of 0.21% indicating that most of the 0.74% thermal expansion mismatch strain was absorbed by plastic deformation. HRXRD full width half maxima values of less than 200 arc sec showed that these layers also had high crystalline quality. Fourier transform infrared transmission measurements at room temperature and 110 K showed absorption edges in the range of 270–80 meV,...

Transfer of PbSe/PbEuSe epilayers grown by MBE on BaF2-coated Si(111)

Abstract Epitaxial growth of PbSe/PbEuSe/BaF 2 /CaF 2 heterostructures was carried out by a molecular beam epitaxy on Si(111) wafers. Successful transfer of 3-μm thick PbSe/PbEuSe epilayers was accomplished by bonding the MBE-grown samples face down to polished copper plates followed by the removal of the silicon substrate by dissolving the BaF 2 buffer layer in water. Nomarski microscopy and surface profile characterization showed that the transferred PbSe/PbEuSe layer had mirror-like morphology. High-resolution X-ray diffraction measurements demonstrated that the PbSe/PbEuSe epilayers maintained high crystalline quality after transfer, and surface analysis by X-ray photoelectron spectroscopy indicated complete removal of BaF 2 buffer layer. The above results indicate that bonded PbSe/PbEuSe could be used as a new substrate material for regrowth of high quality PbSe-related heterostructure devices.

Molecular beam epitaxial growth of IV–VI multiple quantum well structures on Si(111) and BaF2(111) and optical studies of epilayer heating

IV–VI semiconductor multiple quantum well (MQW) structures (PbSe/PbSrSe) were grown on Si(111) and BaF2(111) substrates by molecular beam epitaxy. Structural and optical properties of the MQW structures have been studied using reflection high-energy electron diffraction, high resolution x-ray diffraction (HRXRD), Fourier-transform infrared (FTIR) transmission, and midinfrared photoluminescence (PL). Numerous satellite diffraction peaks and narrow linewidths of the HRXRD rocking curves indicate the high crystalline quality of the structures grown on both Si and BaF2. Longitudinal and oblique valley subband transitions without superposed interference fringes were observed in FTIR differential transmission spectra. Continuous wave midinfrared photoluminescence was also observed from both sets of samples. Comparison of FTIR and PL spectra allows determination of localized epilayer heating in the MQW samples due to optical pumping. For a typical laser pumping power of 9.1 W/cm2 the heating in a MQW layer on Si...