摘要:

Electroluminescence (EL) and photoluminescence have been measured from thin film structures of heavily carbon‐doped GaAs on crystalline semi‐insulating GaAs substrates. EL is only detected when thep‐type film is biased positively. From this and the spectral distribution, it is concluded that there are heterojunction interface states at about 50 meV below the conduction band, with a concentration roughly estimated as 1017cm−3. ©1995 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.113251

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The electrical, optical, and structural properties of light emitting diodes (LEDs) fabricated from the III–V nitride material system have been studied. LEDs with external quantum efficiencies as high as 4% were characterized by transmission electron microscopy and found to contain dislocation densities in excess of 2×1010cm−2. A comparison to other III–V arsenide and phosphide LEDs shows that minority carries in GaN‐based LEDs are remarkably insensitive to the presence of structural defects. Dislocations do not act as efficient nonradiative recombination sites in nitride materials. It is hypothesized that the benign character of dislocations arises from the ionic nature of bonding in the III–V nitrides. ©1995 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.113252

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Microstructure of &agr;‐GaN films grown by organometallic vapor phase epitaxy on sapphire substrates using low temperature AlN (or GaN) buffer layers has been studied by transmission electron microscopy. The defects which penetrate the GaN films are predominantly perfect edge dislocations with Burgers vectors of the 1/3⟨112¯0⟩ type, lying along the [0001] growth direction. The main sources of threading dislocations are the low angle grain boundaries, formed during coalescence of islands at the initial stages of GaN growth. The grain sizes range from 50 to 500 nm, with in‐plane misorientations of less than 3°. The nature of these threading dislocations suggests that the defect density would not likely decrease appreciably at increasing film thickness, and the suppression of these dislocations could be more difficult. ©1995 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.113253

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

In this letter, we present experimental evidence on desorption of O and C from a Si surface resulting in impurity levels below the detection levels of secondary ion mass spectroscopy. We then propose a surface preperation method for silicon epitaxy that consists of anexsituclean and aninsitulow thermal budget prebake in an ultrahigh vacuum rapid thermal chemical vapor deposition (UHV‐RTCVD) reactor. Theexsituclean consists of a standard RCA clean followed by a dilute HF dip and rinse in de‐ionized water. Theinsituclean is either carried out in vacuum or in a low partial pressure of 10% Si2H6in H2. The experiments were conducted in an UHV‐RTCVD reactor equipped with oil‐free vacuum pumps. We propose that the responsible mechanism is desorption of oxygen and hydrocarbons from the Si surface due to the low partial pressures of these contaminants in the growth chamber. If Si2H6is used during the prebake, a sufficiently low growth rate is required in order to provide sufficient time for desorption and avoid Si overgrowth on the O and C sites. ©1995 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.113254

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

In this work, experimentally determined values of electron spectroscopic shifts induced by nitrogen in Ge core levels of substoichiometric amorphous germanium‐nitrogen (a‐GeN) alloys are discussed and presented. X‐ray photoelectron spectroscopy (XPS) and x‐ray excited Auger electron spectroscopy (XAES) are employed to study the behavior of the Ge 3dand LMM spectra, respectively, and combined the corresponding XPS and Auger core levels shifts to determine &Dgr;&agr;’, the modified Auger parameter shift, which is exempt from problems inherent in the interpretation of XPS and XAES shifts. It is demonstrated how one can use &Dgr;&agr;’to reliably estimate &Dgr;nGe, the change in Ge valence charge in the alloys, and how one can calibrate XPS shifts of Si and Ge based alloys in terms of approximate values of &Dgr;nGe. ©1995 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.113255

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

A new theoretical expression is derived for the internal quantum efficiency of solar cells homoepitaxially grown on highly doped monocrystalline substrates. This expression is used to characterize our thin‐layer silicon cells grown by chemical vapor deposition. These cells reach a confirmed efficiency of 17.3% although the active layer thickness is only 48 &mgr;m. The internal quantum efficiency analysis demonstrates that the open circuit voltage is limited by carrier injection into the highly doped substrate. Carrier generation in the substrates accounts for 0.7% of the short circuit current. ©1995 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.113256

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The atomic ordering of GaInP has been established and studied by a variety of methods, including transmission electron microscopy, cathodoluminescence, and photoluminescence. In this work, a Kelvin probe force microscope (KPFM) has been employed to image several GaInP samples previously characterized by these established techniques. The results of our study clearly show that the KPFM is capable of distinguishing between ordered and disordered regions in GaInP, and that the KPFM contrast strongly depends on the amplitude of the applied ac bias voltage of the KPFM. The measurements indicate that ordering in GaInP modifies the density and/or lifetime of the surface states. ©1995 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.113257

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Using a liquid‐nitrogen‐cooled scanning SQUID magnetic microscope, we have developed a technique for broadband imaging of radio‐frequency (rf) and microwave fields with a spatial resolution of about 15 &mgr;m. We have produced images of the amplitude of 50 MHz fields with an rms noise of 2.6 nT and a 300 &mgr;m/s scan rate. Detection is accomplished by using the nonlinearity of the voltage‐flux characteristic of the SQUID to rectify the rf fields. Our present technique is limited by cavity mode resonances in the SrTiO3substrate of our SQUID sensor. Using a small excitation probe, we have directly imaged these resonances at frequencies up to about 12.5 GHz. ©1995 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.113258

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

We present a study of the superconducting morphology of the transport current carrying cross section of a 19‐filament (BiPb)2Ca2Cu3Ox(Bi‐2223) composite conductor using magneto‐optical imaging of magnetic flux patterns. In conjunction with electron microscopy on the same sample this technique allows a unique correlation of superconducting and microstructural properties. Direct evidence for enhanced superconducting properties in platelike regions along the silver/Bi‐2223 interface and for weak properties near the core of the filaments is obtained. Misaligned grain colonies are found to cause an interruption of the superconducting continuity in the filaments. ©1995 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.113259

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Medium energy ion scattering combined with channeling was applied to study the crystalline quality of ultrathin YBaCuO films on (100) SrTiO3and MgO substrates. Films with thicknesses between 3 and 6 nm were deposited by inverted cylindrical magnetron sputtering. Under optimized growth conditionsc‐axis oriented growth was obtained with minimum yield values of 2%–12% (depending on thickness) for films on SrTiO3and 23% on MgO. On the surface of the films a disordered region with a thickness of about 0.6 nm independent of substrate, film thickness, and storage time under ambient conditions was observed. During etching experiments an Y‐oxide layer was formed on the surface. For films on SrTiO3at thicknesses above 4 nm an abrupt increase in the minimum yield was found indicating pseudomorphic growth up to this thickness. In contrast, for films on MgO it was found that a critical film thickness of about 3.6 nm is necessary for the formation of a homogeneous film. ©1995 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.113260

出版商:AIP    

年代:1995

数据来源: AIP