D. V. Lang

Measurement of the band gap of GexSi1−x/Si strained‐layer heterostructures

We have used photocurrent spectroscopy to measure the optical absorption spectra of coherently strained layers of GexSi1−x grown on 〈001〉 Si by molecular beam epitaxy. A dramatic lowering of the indirect band gap, relative to that of unstrained bulk Ge‐Si alloys, is observed. Our results for 0≤x≤0.7 are in remarkably good agreement with recent calculations of the effects of misfit strain on the band edges of coherently strained Ge‐Si heterostructures. At x=0.6, the gap is lower than that of pure Ge.

Modulation doping in GexSi1−x/Si strained layer heterostructures

We report the first observation of the modulation doping effect in Si/Ge0.2Si0.8 heterojunctions grown by molecular beam epitaxy. Peak hole mobilities of ∼3300 cm2 V−1 s−1 have been observed at 4.2 K. These values, although nonoptimum, are comparable to the best reported values for holes in Si/SiO2 inversion layers. Low temperature, angular dependent, Shubnikov–de Haas measurements have demonstrated the two‐dimensional nature of the hole gas and yield a surface carrier density of 3.5×1011 cm−2. From the temperature dependence of the Shubnikov–de Haas amplitudes a hole effective mass of 0.30±0.02mo has been derived. Identical measurements on n‐type heterojunctions having the same Ge content (x=0.2) have failed to show a sustained enhancement of mobility at low temperatures, indicating that ΔEv≫ΔEc.

Measurement of heterojunction band offsets by admittance spectroscopy: InP/Ga0.47In0.53As

We discuss the use of admittance spectroscopy to measure the band offsets of semiconductor heterojunctions. By using this method to analyze the dynamic response of p‐n junctions containing lattice‐matched InP/Ga0.47In0.53As superlattices we can independently determine both the conduction‐ and valence‐band offsets for this materials system. We find that the sum of these offsets equals the known band‐gap difference between InP and Ga0.47In0.53As and that the ratio of the conduction‐band offset to the valence‐band offset is 42:58.

Observation of photoinduced changes in the bulk density of gap states in hydrogenated amorphous silicon

We have observed a reversible photoinduced modification of the bulk density of gap states in a‐Si:H associated with the Staebler–Wronski effect. A detailed numerical analysis of diode admittance and deep level transient spectroscopy measurements shows that the changes in these properties after illumination can be explained by a lowering of the bulk Fermi level and an increase in the density of states below midgap.

Observation of deep levels associated with the GaAs/AlxGa1−xAs interface grown by molecular beam epitaxy

We report the first direct observation of deep levels associated with the n‐GaAs/N‐Al0.25Ga0.75As heterointerface in a double heterostructure (DH) laser grown by molecular beam epitaxy (MBE). The concentration of these states in a nonoptimum device having a high threshold current density is 3×1010 cm−2. The states have an average energy of Ec−0.66 eV and appear to be localized in the n‐GaAs within 140 A of the interface. The relationship of these states to the laser threshold is discussed.

GaAs‐on‐Si: Improved growth conditions, properties of undoped GaAs, high mobility, and fabrication of high‐performance AlGaAs/GaAs selectively doped heterostructure transistors and ring oscillators

Improved growth conditions by molecular‐beam epitaxy (MBE) and fabrication of state‐of‐the‐art AlGaAs/GaAs selectively doped heterostructure transistors (SDHTs) and ring oscillators on Si substrates are reported. In MBE growth, use of minimum As4:Ga flux ratio during initial nucleation combined with in situ thermal cycles gave a marked improvement in material quality. With this method, FWHM of x‐ray rocking curves was measured as low as 135 arcsec for a 3.5‐μm‐thick GaAs layer on Si. Although 3‐μm‐thick undoped GaAs buffer layers on p‐type Si substrates were fully depleted under a Schottky contact, a parallel n‐type conduction path confined in a thin region (<0.1 μm) near the GaAs/Si interface was sometimes observed whose sheet density (1012–1013 cm−2) and mobility (600–900 cm2 V−1 s−1) were independent of temperature between 300 and 77 K. This parallel conduction was successfully prevented by doping 0.1 μm GaAs with 5–10×1016 cm−3 Be atoms near the interface. In AlGaAs/GaAs selectively doped heterostruct...

Electrical activity of defects in molecular beam epitaxially grown GaAs on Si and its reduction by rapid thermal annealing

The electrical activity of defects in GaAs grown on Si by molecular beam epitaxy (MBE) has been examined by studying the diode characteristics and deep level transient spectroscopy (DLTS) of Schottky barriers. The defects are not apparent from the forward bias diode characteristics but they are indicated by large leakage current and early breakdown under reverse‐biased conditions. Post‐growth rapid thermal annealing (RTA) has been found to significantly improve the diode behavior making it almost comparable to GaAs‐on‐GaAs. The reverse current in the as‐grown material shows a very weak temperature dependence, indicating that its origin is not thermionic emission or carrier generation. It is speculated that a large part of this current is due to defect‐assisted tunneling, which is reduced by RTA. DLTS indicated only a modest increase in the concentrations of the well‐known electron traps typical of MBE GaAs with no evidence for new levels in the upper half of the band gap.

Effects of leakage current on deep level transient spectroscopy

A tunnel metal‐insulator‐semiconductor capacitor has been studied by deep level transient spectroscopy (DLTS) technique to demonstrate, along with computer simulation, that the presence of leakage current can drastically affect the DLTS spectra of a broad distribution of states. This leakage current causes carrier capture which competes with the thermal emission to change the trap occupancy. If not recognized and corrected, leakage effects can lead to serious misinterpretations of DLTS results.

Effect of arsenic source on the growth of high‐purity GaAs by molecular beam epitaxy

We report growth of ultrapure GaAs by molecular beam epitaxy (MBE) and show that the arsenic source has a dramatic effect on the purity. With every thing else the same, by changing the arsenic source from 6N grade double refined chunks to a 7N grade slug which closely fits the 35 cm3 crucible, the deep electron trap and the residual acceptor densities were reduced by nearly two orders of magnitude. When intentionally doped with Si to an electron density of 3×1013 cm−3, the measured mobility at 77 K was 205 600 cm2 V−1 s−1, which increased to 294 700 cm2 V−1 s−1 at 42 K. These mobilities are the highest ever observed in intentionally n‐doped MBE GaAs. Low‐temperature photoluminescence studies of both undoped and Si‐doped GaAs layers grown with the As slug show a marked decrease in acceptor‐ and defect‐related luminescence over that observed when the As chunks are used. Our study conclusively proves that the major source of residual acceptor impurities and typical MBE GaAs traps observed by deep level trans...

High photoconductive gain in GexSi1−x/Si strained‐layer superlattice detectors operating at λ=1.3 μm

We demonstrate high photoconductive gain in GexSi1−x/Si strained‐layer superlattice detectors grown by molecular beam epitaxy. The gain mechanism is considered to be the preferential hole trapping in the GexSi1−x wells. Ge0.6Si0.4 waveguide devices operating at the wavelength of 1.3 μm show an optical gain as large as 40 at a low bias of 5–7 V and a gain bandwidth product as large as 3.6 GHz. Preliminary bit error experiments show sensitivity sufficient for transmission at distances of over 25 km and data rates of 200 Mb/s.