D. T. McInturff

Ultrafast carrier dynamics and optical nonlinearities of low‐temperature‐grown InGaAs/InAlAs multiple quantum wells

Low‐temperature‐grown Be‐doped In0.53Ga0.47As/In0.52Al0.48As multiple quantum wells are investigated via wavelength‐dependent time‐resolved nonlinear absorption measurements. Annealed Be‐doped material, in contrast to annealed undoped material, is found to retain the carrier lifetime reduction induced by low‐temperature growth in this narrow‐gap material system. This is attributed to Be–As complexes which, in addition to producing high resistivity material, provide anneal–stable trap states. We also report that ultrafast band‐edge and photoinduced absorption effects can produce subpicosecond absorption recovery in material exhibiting much longer (20 ps) defect‐meditated carrier trapping.

Scanning tunneling microscopy and spectroscopy of arsenic antisites in low temperature grown InGaAs

Scanning tunneling microscopy is used to study low temperature grown (LTG) InGaAs with and without Be doping. The Be-doped material is observed to contain significantly fewer AsGa antisite defects than the undoped material, with no evidence found for Be–As complexes. Annealing of the LTG-InGaAs forms precipitates preferentially in the undoped material. The previously observed dependence of the optical response time on Be doping and annealing is attributed to changes in the As antisite concentration and the compensation effect of the Be.

Use of nonstoichiometry to form GaAs tunnel junctions

A tunnel diode was formed from GaAs containing excess arsenic incorporated by molecular beam epitaxy at reduced substrate temperatures. The incorporation of excess arsenic during growth results in a more efficient incorporation of silicon on donor sites and beryllium on acceptor sites. The better dopant incorporation, along with trap assisted tunneling through deep levels associated with the excess arsenic, results in a tunnel junction with record peak current density of over 1800u2009A/cm2, zero-bias specific resistance of under 1×10−4u2009Ωu2009cm, and a room-temperature peak-to-valley current ratio of 28.

Inhibited oxidation in low‐temperature grown GaAs surface layers observed by photoelectron spectroscopy

The surface oxidation characteristics of a GaAs layer structure consisting of a thin (10 nm) layer of low‐temperature‐grown GaAs (LTG:GaAs) on a heavily n‐doped GaAs layer, both grown by molecular beam epitaxy, have been studied using x‐ray photoelectron spectroscopy (XPS). Between the layer growth and XPS characterization, the unannealed LTG:GaAs sample and a control sample without the LTG:GaAs surface layer were exposed to the atmosphere. The rate of surface oxidation in the sample with a LTG:GaAs surface layer was significantly lower than the oxidation rate of the control sample. This direct observation of inhibited oxidation confirms the surface stability of comparable structures inferred from earlier electrical measurements. The inhibited surface oxidation rate is attributed to the bulk Fermi‐level pinning and the low minority carrier lifetime in unannealed LTG:GaAs.

Stability of a low‐temperature grown GaAs surface layer following air exposure using tunneling microscopy

The stability of a GaAs layer structure consisting of a thin (10 nm) layer of low‐temperature‐grown GaAs on a heavily n‐doped GaAs layer, both grown by molecular beam epitaxy, has been studied using a scanning tunneling microscope. The sample was exposed to the atmosphere between the layer growth and STM characterization. Tunneling spectroscopy shows both the GaAs band edges and a band of midgap states associated with the excess As in the surface layer. The observation of midgap states following atmospheric exposure indicates that the low‐temperature‐grown GaAs layer does not oxidize rapidly. The spectroscopy results are used to confirm a model for conduction in low resistance, nonalloyed contacts employing comparable layer structures.

An ohmic nanocontact to GaAs

The formation and characterization of nanometer-size, ohmic contacts to n-type GaAs substrates are described. The nanocontacts are formed between a single-crystalline, nanometer-size Au cluster and a GaAs structure capped with layer of low-temperature-grown GaAs (LTG:GaAs). An organic monolayer of xylyl dithiol (p-xylene-α,α′- dithiol; C8H10S2) provides mechanical and electronic tethering of the Au cluster to the LTG:GaAs surface. The I(V) data of the Au cluster/xylyl dithiol/GaAs show ohmic contact behavior with good repeatability between various clusters distributed across the surface. The specific contact resistance is determined to be 1×10−6 Ωu200acm2. Current densities above 1×106 A/cm2 have been observed.

Magnetic and magnetoresistance measurements on iron-based nanoclusters in In0.53Ga0.47As

We have obtained magnetoresistance data on low iron-concentration samples (∼1%) showing a large negative magnetoresistance (3.2% at 5 K in 0.5 T) attributed to imbedded superparamagnetic clusters in In0.53Ga0.47As. The samples were prepared by ion implanting a p-In0.53Ga0.47As layer with iron followed by a rapid thermal anneal. Magnetic measurements confirm the formation of a cluster size distribution with a mean diameter of 6.2 nm and effective moment of 7000 bohr magnetons. The magnetization of these single domain ferromagnets is 50% saturated in a field of only 0.2 T even at room temperature which is important for device applications.

Reflection anisotropy spectroscopy study of the near surface electric field in low-temperature grown GaAs (001)

We have evaluated an “effective depletion width” of ⩽45 A and the sign (n-type/upward band bending) of the near surface electric field in low-temperature grown GaAs (001) using the optical method of reflection anisotropy spectroscopy in the vicinity of the spin-orbit split E1,u2002E1+Δ1 optical features. Our results provide evidence that surface Fermi level pinning occurs for air exposed (001) surfaces of undoped low temperature grown GaAs.

Temperature-dependent behavior of low-temperature-grown GaAs nonalloyed ohmic contacts

A study of nonalloyed ohmic contact structures consisting of Au/Ti metallization deposited on a thin (3.5–5 nm) layer of low-temperature-grown GaAs (LTG:GaAs) on a thin (10 nm) layer of heavily doped n-type GaAs is summarized. We demonstrate that this Au/Ti:LTG:GaAs/n+GaAs contact structure has a stable specific contact resistance between 40 and 300 K, with measured contact resistance as low as 2×10−6u200aΩu200acm2 at 40 K. Based on comparisons of the measured data with calculations using a uniformly doped Schottky model, we infer that the activation doping density in these structures is higher than 5×1018u200acm−3, and that the surface potential barrier height is lower than 0.7 eV (midgap). The characteristic current–voltage curves of the nonalloyed contact show that tunneling is the primary conduction mechanism.

The compensation and depletion behavior of iron doped GaAs grown by molecular beam epitaxy

We describe the growth and characterization of GaAs films in which a high concentration (1 at.u2009%) of elemental iron is introduced during growth in a conventional molecular beam epitaxy system. For films grown at 600u2009°C, the iron incorporates as Fe3GaAs precipitates. Unlike the formation of As precipitates in low temperature grown (LTG) GaAs, iron precipitate formation does not require a postgrowth anneal. Hall measurements of the as‐grown GaAs/Fe3GaAs composite intentionally doped with silicon indicate that Fe3GaAs precipitates will deplete carriers in the same manner as As precipitates deplete carriers in annealed LTG GaAs. The degree of depletion depends on the initial growth temperature and the intentional doping level. Electrical behavior of samples subjected to a postgrowth rapid thermal anneal indicate that the material is deep level compensated by iron acceptor doping via dissolution of the Fe3GaAs precipitates.