N. M. Haegel

Reactive ion etching of GaAs with CCl2F2:O2: Etch rates, surface chemistry, and residual damage

The reactive ion etching of GaAs with a CCl2F2:O2 discharge was investigated as a function of gas flow rate (10–60 sccm), total pressure (2–50 mTorr), power density (0.25–1.31 W cm−2), gas composition (0%–70% O2), and etch time (1–64 min). The etch rate decreases with increasing gas flow rate, increases with increasing power density, and goes through a maximum at a gas composition of 75:25 CCl2F2:O2 under our conditions. After etching at low‐power densities (0.56 W cm−2) and for high CCl2F2 ratios (19:1 to O2), carbon and chlorine could be detected in the GaAs to a depth of less than 15 A by x‐ray photoelectron spectroscopy. Under these conditions there was a Ga deficiency to a depth of ∼100 A, which we ascribe to surface roughening and the preferential vaporization of As2O3 over Ga2O3. At high‐power densities (1.31 W cm−2) a polymeric layer several hundred angstroms thick containing CCl and CF bonds was observed on the GaAs surface. Etching under O2‐rich conditions did not lead to any additional creation...

Interpretation of photoluminescence excitation spectroscopy of porous Si layers

Photoluminescence excitation (PLE) measurements allow one to obtain direct information on the absorption processes in porous Si leading to the strong visible luminescence which has recently been reported. This technique does not need a free‐standing porous Si sample, but the effect of layer thickness should be included in the interpretation of PLE data. In our PLE spectra, two separate absorption edges are observed and the initial one shifts to higher energies with storage time. The trends of these changes are consistent with the quantum wire model.

Characterization of solution-synthesized CdTe and HgTe

We report the characterization of solution-synthesized CdTe and HgTe nanocrystals by X-ray diffraction, transmission electron microscopy, and photoluminescence. Methanol solutions of sodium telluride and cadmium iodide or mercury iodide, respectively, are reacted to precipitate the nanocrystalline metal tellurides, while the sodium iodide byproduct remains in solution. The existence of crystalline CdTe, HgTe, and ternary HgCdTe compounds has been demonstrated by powder X-ray diffraction after a post-synthesis sintering process. Precipitated crystallites from this synthesis were analyzed by transmission electron microscopy, which revealed that crystal diameters can vary from approximately 1 nm to 100 nm and that crystals are stoichiometric within the detection limit of the electron microprobe technique. Narrow size ranges can be selected and investigated due to an in-situ separation process in the electron microscope. Photoluminescence is found at energies above the bulk exciton energy for CdTe and is attributed to near-band-gap recombination which is blue-shifted due to quantum confinement. Both low defect luminescence and dark field imaging suggest a high crystalline quality. A comparative characterization by photoluminescence, transmission electron microscopy, and X-ray diffraction evaluates the effects of heat treatments during and after synthesis.

RELAXATION SEMICONDUCTORS : IN THEORY AND IN PRACTICE

Relaxation semiconductors are materials dominated by free carrier transport and defined by the condition that the dielectric relaxation time τD is longer than the free carrier lifetime τ0. Novel transport behavior has been demonstrated, both theoretically and experimentally, to be associated with this regime of semiconductor behavior. This review surveys the history of the field, emphasizes recent experimental and modeling work and summarizes our current understanding of relaxation behavior in crystalline semiconductors.

Spatial distribution of free‐carrier lifetime and deep‐level luminescence across a semi‐insulating GaAs wafer

The free‐carrier lifetime, the near band‐edge luminescence (band‐to‐band and band‐to‐acceptor transitions), and the near‐infrared deep‐level luminescence (two bands at 0.7 and 0.8 eV) are mapped across an undoped semi‐insulating GaAs wafer. The carrier lifetime follows on a large scale a W‐shaped profile across the wafer with lifetimes varying between 200 and 500 ps at T=5 K. The band‐to‐band, band‐to‐acceptor, and the 0.7‐eV luminescence are intense wherever the lifetime is long. However, the intensity of the 0.8‐eV luminescence is high where the lifetime is short. The intensity variation of the 0.8‐eV luminescence could quantitatively explain the lifetime distribution. Comparison with near‐infrared absorption data show that EL20 as mapped by near‐infrared absorption is not responsible for the short lifetime. Additionally, the lifetime shows a short‐distance variation with a cell size of about 300 μm.

Characterization of InP/GaAs/Si structures grown by atmospheric pressure metalorganic chemical vapor deposition

The thickness dependence of material quality of InP‐GaAs‐Si structures grown by atmospheric pressure metalorganic chemical vapor deposition was investigated. The InP thickness was varied from 1–4 μm, and that of the GaAs from 0.1–4 μm. For a given thickness of InP, its ion channeling yield and x‐ray peak width were essentially independent of the GaAs layer thickness. The InP x‐ray peak widths were typically 400–440 arcsec for 4‐μm‐thick layers grown on GaAs. The GaAs x‐ray widths in turn varied from 320–1000 arcsec for layer thicknesses from 0.1–4 μm. Cross‐sectional transmission electron microscopy showed high defect densities at both the InP‐GaAs and GaAs‐Si interfaces. In 4‐μm‐thick InP layers the average threading dislocation density was in the range (3–8)×108 cm−2 with a stacking fault density within the range (0.4–2)×108 cm2. The He+ ion channeling yield near the InP surface was similar to that of bulk InP (χmin∼4%), but rose rapidly toward the InP‐GaAs heterointerface where it was typically around ...

Transport Imaging for Contact-Free Measurements of Minority Carrier Diffusion in GaN, GaN/AlGaN, and GaN/InGaN Core-Shell Nanowires

Minority carrier diffusion lengths (Ld) are measured for GaN, GaN/AlGaN, and GaN/InGaN core-shell nanowires using a technique based on imaging of recombination luminescence. The effect of shell material on transport properties is measured. An AlGaN shell produces Ld values in excess of 1 μm and a relative insensitivity to wire diameter. An InGaN shell reduces effective diffusion length, while a dependence of Ld on diameter is observed for uncoated nanowires.

Direct imaging of anisotropic minority-carrier diffusion in ordered GaInP

An all-optical technique has been used to provide the first direct measurement of anisotropic minority-carrier diffusion in an ordered alloy of GaInP. Direct imaging of the minority-carrier diffusion distribution resulting from generation at a quasipoint source is obtained using an optical microscope coupled to a scanning electron microscope. Minority-carrier diffusion lengths ranging from 3 to 60 μm are measured by this technique in double heterostructures of GaInP, GaAs, and GaInAs, providing a key parameter of interest to the performance of state-of-the-art triple junction solar cells. Here we show a direct measurement of anisotropy in minority-carrier mobility in ordered GaInP, which is evident in the oval-shaped distribution of the recombination luminescence. A factor of 1.6 increase in minority electron mobility along the [110] major axis is reported.

Enhanced hot‐electron photoluminescence from heavily carbon‐doped GaAs

An enhancement of hot‐electron photoluminescence due to degenerate conditions in the valence band has been observed in metalorganic molecular beam epitaxial grown GaAs:C with net acceptor concentration of up to 4×1020 cm−3. The photoluminescence (PL) was studied as a function of free‐carrier concentration and sample temperature. Comparison of the PL spectra from the heavily doped GaAs to that of undoped material shows a peak shift to lower energy coupled with a greatly enhanced high‐energy tail extending into the visible region of the spectrum. At 300 K, luminescence at 1.8 eV is observed at 20% the intensity of the peak luminescence at 1.36 eV.

Imaging transport for the determination of minority carrier diffusion length

A scanning electron microscope technique is used, in combination with an optical imaging system, to measure minority carrier diffusion length in a heavily doped GaAs double heterostructure. Diffusion and drift of charge are imaged. A diffusion length of 3.6μm is measured, corresponding to a minority carrier mobility of 1150cm2∕Vs in p-type material doped ∼5×1018cm−3. Measurements are made as a function of local electric field and sample temperature. The technique offers a flexible approach to direct measurement of transport properties and is applicable to a range of luminescent materials and multilayer devices.