D. E. Mars

Cathodoluminescence atomic scale images of monolayer islands at GaAs/GaAlAs interfaces

Direct images of growth islands differing by 2.8 A [1 monolayer (ML)] height at GaAs/AlGaAs heterointerfaces and of the columnar structure of quantum wells are reported for the first time. The structures are grown by molecular‐beam epitaxy (MBE) with interruptions of the growth of ≊2 min at the interfaces. The method used to obtain these images is scanning cathodoluminescence. The dependence of the lateral extension of these islands on growth conditions is investigated. For fixed growth rate rs≊0.5 ML/s the mean island size decreases from 6–7 μm to 2 μm upon an increase of growth temperature from Tg=600 to 660 °C. Apparently the growth process changes from a planar to a three‐dimensional one. For low‐growth temperature and rate the lateral extension of such islands can be larger than the carrier diffusion length. Under these conditions interisland thermalization of carriers is largely suppressed. Quantitative information on the reduction of roughness of the quantum well interfaces with increasing growth i...

Structural changes of the interface, enhanced interface incorporation of acceptors, and luminescence efficiency degradation in GaAs quantum wells grown by molecular beam epitaxy upon growth interruption

A comparison of the luminescence of GaAlAs/GaAs/GaAlAs quantum wells (QW) grown by molecular beam epitaxy with and without a 1–100 s interruption of the growth at the interfaces is presented. Well widths of 2 and 5 nm are studied as model systems. The luminescence from the noninterrupted samples consists of Gaussian shaped doublets. A line shape theory is outlined, allowing for the first time a determination of the distribution of the microscopic chemical and crystallographic disorder of the interfaces from the spectra. All samples were grown under nominally identical conditions and all show the same Gaussian distribution of the interface position. The full width at half maximum of the distribution function is 1.25 A. The interfaces in these samples are believed to have an island‐like character with a typical island size of much less than 17 nm. Interruption of the growth by a few seconds changes the luminescence line shape function qualitatively. The spectrum splits into doublets, with doublet fine struc...

Interband optical absorption in free standing layer of Ga0.96In0.04As0.99N0.01

We have measured the interband optical absorption of a free-standing sample of Ga0.96In0.04As0.99N0.01 in a wide energy range from 1 to 2.5 eV. We found that the fundamental absorption edge is shifted by 150 meV towards lower energies, and the absorption coefficient measured at higher energies exhibits substantial reduction comparing to that of GaAs. By removing the GaAs substrate, we were able to get an experimental insight into the interband optical transitions and the density of state in this material. The changes can be understood within the band anticrossing model predicting the conduction band splitting. New absorption edges associated with optical transitions from the spin-orbit split off band to the lower conduction subband (1.55 eV) and from the top of the valence band to the upper subband (1.85 eV) are observed.

Pressure and temperature dependence of the absorption edge of a thick Ga0.92In0.08As0.985N0.015 layer

We have studied the pressure and temperature dependence of the absorption edge of a 4-μm-thick layer of the alloy Ga0.92In0.08As0.985N0.015. We have measured the hydrostatic pressure coefficient of the energy gap of this alloy to be 51 meV/GPa, which is more than a factor two lower than that of GaAs (116 meV/GPa). This surprisingly large lowering of the pressure coefficient is attributed to the addition of only ∼1.5% nitrogen. In addition, the temperature-induced shift of the edge is reduced by the presence of nitrogen. We can explain this reduction by the substantial decrease of the dilatation term in the temperature dependence of the energy gap.

Growth of 1.3 μm InGaAsN laser material on GaAs by molecular beam epitaxy

We have grown bulk GaAsN and InGaAsN quantum well laser structures using molecular beam epitaxy and an electron cyclotron resonance plasma source with N2 gas. X-ray diffraction measurements in GaAsN grown on GaAs were used to determine the concentration of N in the range of 0% to ∼2%. Room temperature photoluminescence (PL) measurements were done on quantum well test structures and half lasers. The PL intensity decreases and the PL full width at half maximum (FWHM) increases as the wavelength increases. Rapid thermal annealing (RTA) at 850 °C for 10 s improves the PL intensity by a factor of 8 and increases the PL peak emission energy by 80 meV. The longest wavelength measured to date in laser structures with single quantum wells of InGaAsN is 1480 nm with a FWHM of 60 meV. Samples with and without RTA were fabricated into broad-area lasers with dimensions of 50×500 μm2. Laser devices with RTA operated in the pulsed mode at 1.3 μm with a threshold current density of 9.5 kA/cm2.

Heterojunction‐induced phenomena in Hall effect and photoconductivity measurements of epitaxial AlxGa1−xAs

Temperature‐dependent Hall effect measurements are reported for n‐type, Si‐doped Al0.32Ga0.68As films grown on semi‐insulating GaAs substrates. Results are presented for films grown with and without undoped Al0.32Ga0.68As buffer layers. For T≥200 K, the free electron concentration in these Si‐doped films has an exponential dependence on temperature with an activation energy of ∼93 meV. The temperature‐independent free‐electron concentration observed for T≤100 K is due to conduction in the GaAs substrate near the substrate‐epilayer interface (i.e., a two‐dimensional electron gas). Hall mobilities measured in thin (≤5 μm) AlxGa1−xAs films, particularly at low temperatures, can be dominated by this conduction mechanism and are thus invalid as a measure of the transport properties of the AlxGa1−xAs films. In addition, photoconductivity data are presented which show that the persistent photoconductivity effect observed in these films is due primarily to charge separation at the AlxGa1−xAs/GaAs heterojunction.

Anomalies in MODFET's with a low-temperature buffer

GaAs buffer layers grown by molecular-beam epitaxy (MBE) at low temperatures (200-300 degrees C) have been successfully used to reduce sidegating in both MESFETs and MODFETs. There are, however, high concentrations of defects in the low-temperature (LT) buffers that adversely affect the high-frequency performance of precision analog and certain digital circuits. In unoptimized structures, nanosecond and microsecond transients are as large as 85 and 15% of the total voltage swing, respectively. These transients cause various detrimental effects in circuits. These effects are described. Their origin is attributed to the outdiffusion of defects from the LT buffer, and a method for optimizing the device structure for minimum sidegating and maximum high-frequency performance is presented. >

Deep electron traps in organometallic vapor phase grown AlxGa1−xAs

Deep electron traps have been studied by means of deep level transient spectroscopy in n‐type nominally undoped and intentionally Te‐doped AlxGa1−xAs epitaxial layers which were grown by vapor phase epitaxy from organometallic compounds (OMVPE). Three main deep electron levels are present in undoped material: a trap with an activation energy of 0.8 eV, which is also found in GaAs grown by conventional VPE, and two levels specific to OMVPE with activation energies of 0.32 and 0.38 eV, respectively. The concentration of the 0.8 eV level is found to be independent of the aluminum content x, supporting the assumption that it is not related to substitutional oxygen. The other levels, however, exhibit a very strong dependence of concentration on the composition, varying by four orders of magnitude in the range of 0⩽x⩽0.35. In Te‐doped samples, a level with an activation energy of 0.23 eV has been identified, which is thought to be related to an IR emission found in photoluminescence in OMVPE as well as in liqui...

In situ thickness monitoring and control for highly reproducible growth of distributed Bragg reflectors

A theoretical model was developed to simulate the apparent substrate temperature oscillation during the growth of AlAs/AlxGa1−xAs, x=0 and 0.25, distributed Bragg reflectors (DBR) for 980‐ and 780‐nm vertical cavity surface emitting lasers, respectively. The simulated data were then used for in situ monitoring and feedback control of layer thickness by a simple pyrometric interferometry technique to obtain a highly reproducible DBR. These measurements can be performed with continuous substrate rotation and without any growth interruption. The reproducibility of the center wavelength and full width at half‐maximum of the reflectivity stop band with a variation of <±0.2% and <±0.4% for the AlAs/GaAs and AlAs/AlGaAs mirror stacks, respectively, were achieved.

Analysis of wafer fusing for 1.3 μm vertical cavity surface emitting lasers

We report low densities of electrically active defects and low optical losses at the wafer fused interface between InP and GaAs. Electron beam induced current analysis shows electrically active defects with an average spacing of 4.5 μm at the interface and significantly lower densities 0.4 μm from the fused interface. Optical measurements of a Fabry–Perot resonator made by fusing an InP epilayer to a GaAs/AlAs mirror demonstrate a 3% increase in mirror transmission after fusing and negligible absorption at the fused interface. Based on these results, we present design considerations for fused surface emitting lasers.