James A. Hutchby

Photoluminescence of ion‐implanted GaN

Thirty‐five elements were implanted in GaN. Their photoluminescence spectra were measured and compared to those of an unimplanted control sample. Most impurities emit a peak at about 2.15 eV. Mg, Zn, Cd, Ca, As, Hg, and Ag have more characteristic emissions. Zn provides the most efficient recombination center. A set of midgap states is generated during the damage‐annealing treatment.

Theoretical analysis of AlxGa1−xAs‐GaAs graded band‐gap solar cell

A practical theoretical analysis of an n/p graded band‐gap AlxGa1−xAs‐GaAs solar cell indicates that the presence of a built‐in electric field acting on holes in the surface layer increases the hole collection efficiency of a nearly optimum cell to a maximum of 97.8%. The electric field is created by the band‐gap gradient and serves to reduce the surface hole recombination by 97% and reduce the bulk hole recombination by 80%, compared to a similar GaAs cell. These reduced losses increase cell response substantially for wavelengths less than 0.59 μm and yield a maximum air‐mass‐zero efficiency of 17.7% (not corrected for a 13% front‐surface contact). The model includes an optimized antireflection coating, series resistance, and junction‐recombination current.A practical theoretical analysis of an n/p graded band‐gap AlxGa1−xAs‐GaAs solar cell indicates that the presence of a built‐in electric field acting on holes in the surface layer increases the hole collection efficiency of a nearly optimum cell to a maximum of 97.8%. The electric field is created by the band‐gap gradient and serves to reduce the surface hole recombination by 97% and reduce the bulk hole recombination by 80%, compared to a similar GaAs cell. These reduced losses increase cell response substantially for wavelengths less than 0.59 μm and yield a maximum air‐mass‐zero efficiency of 17.7% (not corrected for a 13% front‐surface contact). The model includes an optimized antireflection coating, series resistance, and junction‐recombination current.

Photoluminescence of Zn‐implanted GaN

The photoluminescence spectrum of Zn‐implanted GaN peaks at 2.87 eV at room temperature. The emission efficiency decreases linearly with the logarithm of the Zn concentration in the range 1–20×1018 Zn/cm3.

High−efficiency graded band−gap AlxGa1−xAs−GaAs solar cell

A detailed theoretical analysis of an n−on−p graded band−gap AlxGa1−xAs−GaAs solar cell yields a maximum air mass zero power conversion efficiency of 17% compared to 9% for a similar GaAs cell. The analaysis includes surface and bulk minority carrier recombination, junction recombination current, spectrally varying surface reflection, and series resistance loss. The maximum efficiency is determined for a surface recombination velocity of 1×105 cm/sec and hole and electron diffusion lengths of 2.1 and 7.6 μm, respectively. The improved efficiency is primarily due to a built−in electric field, caused by the band−gap gradation, accelerating photogenerated holes toward the p−n junction. This field reduces the surface and bulk recombination of the holes, and thereby enhances their collection.

Theoretical optimization and parametric study of n‐on‐p AlxGa1−xAs‐GaAs graded band‐gap solar cell

A comprehensive theoretical model of the graded band‐gap AlxGa1−xAs‐GaAs solar cell is used to optimize the n‐on‐p cell. The model includes power losses due to surface, bulk, and junction minority‐carrier recombination, series resistance, and photon reflection from an SiO antireflection coating of optimum thickness. The optimized cell has a junction depth/graded band‐gap layer thickness of 1.0 μm, respective donor and acceptor concentrations of 4×1017 and 2×1017 cm−3, and a surface AlAs mole fraction of x=0.35. The optimized graded band‐gap cell has an air‐mass‐zero efficiency of 17.7% (not corrected for a 13% front surface contact area) and is shown to be less sensitive than a similar n‐on‐p GaAs cell to material degradation in the form of decreased minority‐carrier diffusion lengths and increased surface‐recombination velocity.

Temperature dependence of electrical properties in Be‐implanted semi‐insulating GaAs

The Hall coefficient and sheet resistivity have been measured as functions of temperature for Cr‐doped semi‐insulating GaAs implanted with 100‐keV Be ions at fluences between 1×1013 and 5×1015 cm−2. Impurity conduction influences or dominates hole transport processes at all temperatures for fluences of 1×1014, 4×1014, and 5×1015 cm−2, but is evident only for T <80 °K for lower fluence implants of 1×1013 and 4×1013 cm−2. Analysis of higher‐temperature sheet‐hole concentration data for these lower‐fluence implants indicates the implanted layers are heavily doped and compensated, and the activation energy Ea decreases with increasing ionized acceptor concentration Nai. These results, along with an earlier result for a much lower concentration of implanted Be acceptors, can be represented by Ea=28.4 meV −(1.77×10−5) N1/3ai. In addition, measurement of electrical properties as a function of room‐temperature sample‐storage time indicates slow activation of compensating donors for the 4×1013‐cm−2 implant. This e...

Optical reflection studies of lattice disorder in iodine implanted CdS.

Abstract Optical reflectivity spectra (3.45 to 5.90 eV) of CdS implanted with 40 keV I+ are presented for doses between 5 × 1013 and 1 × 1016 ions/cm2 and for isochronal anneals up to 550°C. Typical peaks (E 1: A, B) present in crystalline CdS decrease with increasing dose until they are no longer resolvable; however, a prominent saturation observed in Si and somewhat in GaAs is not found for doses up to 1 × 1016 ions/cm2. Correlation with similar results obtained from the Rutherford scattering technique suggests the reflection variations are damage related and verifies previous results that anobservable amorphous layer is not formed. Annealing tends to increase the peaks with complete restoration occurring for doses 7 × 1014 ions/cm2 annealed at 500°C.

Transport of minority-carriers in graded-composition semiconductors and its impact on graded-band-gap base transistors operated at high currents

A comprehensive model for minority‐carrier transport due to a quasi‐electric‐field in graded‐composition semiconductor materials is developed including effects due to nondegenerate high‐level injection of excess electrons and holes. For low‐level injection, equivalent to a one‐sun photoexcitation of uniformly doped direct‐ or indirect‐band‐gap material the total quasifield is given by the standard gradient of band‐gap energy. High‐level injection asymptotically reduces quasifield components due to band‐gap and effective‐mass gradients to lower values, and essentially eliminates components due to equilibrium‐ and excess‐carrier concentration gradients. The impact of these results on performance of p‐n‐p and n‐p‐n graded‐band‐gap base transistors operated at high currents is examined.

Comparative radiation resistance calculation for graded‐ and constant‐composition n AlxGa1−xAs–p AlzGa1−zAs solar cells

The performance and radiation resistance of a new double‐graded‐band‐gap n AlxGa1−xAs–p AlzGa1−zAs solar cell are theoretically determined. The performance of this device is similar to that of the single‐graded‐band‐gap cell. The power‐conversion efficiencies of both graded‐band‐gap structures are shown to be less sensitive to minority‐carrier lifetime degradation than a similar constant‐composition n AlxGa1−xAs–p GaAs heteroface cell.

Stopping cross sections for 0.25–3.0‐MeV 4He ions in cadmium sulfide

Stopping cross sections of 4He ions with energies 0.25–3.0 MeV have been measured for cadmium sulfide with a probable error of ±7–8%. The experimental method utilized the Rutherford backscattering technique and measured the energy loss of elastically scattered 4He ions from films of cadmium sulfide sputtered on carbon substrates. The experimental data are compared to recent experimental and theoretical results.