K. Sadra

Scanning tunneling microscopy of doping and compositional III–V homo‐ and heterostructures

Scanning tunneling microscopy (STM) was used to study the (110) cross‐sectional surfaces of molecular‐beam epitaxially grown III–V homo‐ and heterostructures, which include GaAs multiple p–n junctions, (InGa)As/GaAs strained‐layer multiple quantum wells, and (AlGa)As/GaAs heterojunctions. Both doping and compositional effects can be resolved by the topographic contrasts of constant‐current STM images. The samples were prepared by either cleaving in ultrahigh vacuum or cleaving ex situ followed by sulfide [(NH4)2S] passivation. Sulfide passivated samples have been found to be advantageous for the measurements of scanning tunneling spectroscopy.

Scanning tunneling microscopy of GaAs multiple pn junctions

Scanning tunneling microscopy and spectroscopy is used to study GaAs multiple pn junction samples cleaved in ultrahigh vacuum. Direct topographic contrast over the pn junctions can be observed in the constant current imaging mode. The topographic height in the p‐type regions appears much lower (by about 5 A) than that in the n‐type regions. Tunneling spectroscopy measurements show consistency with the assignment of the p‐ and n‐type regions. We discuss a possible mechanism for the observed contrast.

Relationship between photoluminescence spectra and low‐field electrical properties of modulation‐doped AlGaAs/GaAs quantum wells

We describe the influence of sheet charge density and crystalline quality on the photoluminescence (PL) spectra of AlGaAs/GaAs n‐type modulation doped quantum wells (MDQWs). We discuss the various contributions to the PL linewidth at 4.2 K. The linewidth at 77 K is approximately equal to the Fermi energy, and is independent of crystalline quality, making it a good measure of sheet carrier density. At 4.2 K, the crystalline quality also influences the PL linewidths; however, the carrier density can be deduced from the high‐energy cutoff point of the PL spectra. The ratio of 77 K to 4.2 K linewidths correlates fairly well with the crystalline quality, as measured by the 77 K Hall mobility. Our calculations of the band diagram, wavefunctions, and carrier densities provide a deeper understanding of these structures. Results of this work have applications in nondestructive testing of large area wafers for uniformity in sheet carrier density and mobility as well as in the design of novel optoelectronic devices.

Comparative study of cross‐sectional scanning tunneling microscopy/spectroscopy on III–V hetero‐ and homostructures: Ultrahigh vacuum‐cleaved versus sulfide passivated

A comparative study of surfaces prepared by sulfide passivation and by UHV cleaving using cross‐sectional scanning tunneling microscopy and spectroscopy (XSTM/S) is performed. Test samples used include both GaAs/(AlGa)As heterojunctions and GaAs pn junctions. Sulfide‐ passivated heterojunction surfaces allow much useful electronic information to be deduced from the tunneling spectroscopy since the tip‐induced band bending problem is solved. Atomic resolution across UHV‐cleaved heterojunctions allows a direct measurement of the asymmetrical interfacial roughness which agrees very well with the value deduced from tunneling spectroscopy on the sulfide‐passivated surface. In the case of the UHV‐cleaved pn junction surface, the tip‐induced band bending effect is seen to manifest itself as a spatial shift in the conductivity profile within the depletion region. Sulfide‐passivated pn junctions show a topographic profile which correlates very well with the secondary ion mass spectrometry profile, indicating that ...

Effects of multiband electron-hole scattering and hole wave-function symmetry on minority-electron transport in GaAs

A Monte Carlo investigation of the effect of multiband electron‐hole scattering on steady‐state minority‐electron transport in room‐temperature p‐GaAs is reported. A comparative study of the strength of such scattering mechanisms is presented and shows that a careful treatment of these processes is indispensible for a quantitative microscopic understanding of minority‐electron behavior. The effect of the p‐type symmetry of hole wave functions on multiband processes and minority‐electron transport is also described. Finally, we discuss several additional issues, which we believe must be carefully investigated before present Monte Carlo treatments of electron‐hole scattering can provide reliable quantitative information.

A Monte Carlo study of electron‐hole scattering and steady‐state minority‐electron transport in GaAs

We report the first bipolar Monte Carlo calculations of steady‐state minority‐electron transport in room‐temperature p‐GaAs including multiband electron‐hole scattering with and without hole overlap factors. Our results show how such processes, which make a significant contribution to the minority‐electron energy loss rate, can affect steady‐state minority‐electron transport. Furthermore, we discuss several other issues which we believe should be investigated before present Monte Carlo treatments of electron‐hole scattering can provide quantitative information.

CROSS-SECTIONAL SCANNING TUNNELING MICROSCOPY AND SPECTROSCOPY OF PASSIVATED III-V HETEROSTRUCTURES

Structural and electronic properties of Al0.3Ga0.7As/GaAs heterojunction and GaAs pn junction systems are investigated by cross‐sectional scanning tunneling microscopy and spectroscopy (XSTM/S). The cross‐sectional samples were prepared by passivating ex situ with a sulfide [(NH4)2S] solution and were transferred into an ultra‐high vacuum system for STM/S studies. It is found that passivated samples are advantageous for the measurements of scanning tunneling spectroscopy. The STM/S results and the experimental details are reported.

The role of the split-off band in electron-hole energy exchange dynamics in selected III-V semiconductors

We report a Monte‐Carlo investigation of Coulomb electron‐hole scattering processes involving the split‐off band in selected III‐V semiconductors in the context of steady‐state high‐field minority‐electron transport. Due to the small value of the relevant hole overlap factors, as well as the relative values of the Γ‐L energy separation and the split‐off energy, such processes do not make a significant contribution to the minority electron energy loss rate in In0.53Ga0.47As. In InP, however, the ΓhΓs process accounts for a significant fraction of the total Γ‐electron energy loss to holes.

Drift and diffusion in low‐dimensional p‐n junctions

We present a theoretical drift‐and‐diffusion study of the current‐voltage characteristics of low‐dimensional p‐n junctions. Under low‐level injection, low‐dimensional p‐n junctions exhibit current‐voltage characteristics similar to those of three‐dimensional diodes. Under high‐level injection, the dependence of minority‐carrier diffusion coefficients on the low‐dimensional density of states may lead to the appearance of features in the small‐signal properties. Such features become more pronounced as the temperature is lowered.

Electron-Hole Scattering And Minority-Electron Transport in In 0 . 53 Ga 0 .47 As, InAs, and InP: The Role Of The Split-Off Band

We report a Monte-Carlo investigation of Coulomb electron-hole scattering processes involving the split-off band in selected III-V semiconductors in the context of steady-state high-field minority-electron transport. Due to the small value of the relevant hole overlap factors, as well as the relative values of the r-L energy separation and the split-off energy, such processes do not make a significant contribution to the minority electron energy loss rate in In0.53Ga0.47As. In InP, however, the rhrs process accounts for a significant fraction of the total r-electron energy loss to holes.