R. H. Williams

Growth of InxGa1−xAs on GaAs (001) by molecular beam epitaxy

Abstract As a precursor to investigating the growth of In x Ga 1 − x As on Si, a series of 2.8 ± 0.2 μ m thick films of various compositions ( x = 0, 0.13, 0.56 and 1.0) have been deposited by molecular beam epitaxy onto (001) GaAs on-axis substrates, using a minimized As 4 flux and a range of growth temperatures. In addition, a series of layers were grown at a fixed temperature of 350°C over the whole composition range. It was found that the electrical and structural characteristics of InAs and In 0.56 Ga 0.44 As were very similar, in both cases the layers were n-type, and growth below 330° produced a rapid deterioration of crystalline quality with the associated dislocations/defects being electrically active. In addition, the characteristics of GaAs and In 0.13 Ga 0.87 As were also found to be very similar. Both were doped with Si to make them conduct and showed a transition to insulating behaviour on reducing the growth temperature below ∼ 480±10°C; above this temperature good mobilities were obtained. Increasing the growth temperature for all the In containing alloys produced a gradual increase in surface roughness up to the point at which In accumulation on the surface occurred (∼410°C for InAs, ∼510°C for In 0.56 Ga 0.44 As and ∼550°C for In 0.13 Ga 0.87 As). Growth at 350°C showed a rapid deterioration in crystalline quality as x approached 0.5, as can be illustrated by a pronounced maximum in the X-ray diffraction peak width.

Surface reconstructions of GaAs(111)A and (111)B: A static surface phase study by reflection high‐energy electron diffraction

GaAs(111)A and (111)B static surface phase maps have been generated under a variety of substrate temperature and incident As4 flux conditions ranging from, respectively, 400–700 °C, and from 1×1014 to 1×1016 molecules cm−2 s−1. For the case of GaAs(111)A only a (2×2) reconstruction was observed. However, four GaAs(111)B surface reconstructions were identified below a critical As4 flux of JcAs4≂5×1015 molecules cm−2 s−1, viz.: (2×2); (1×1)LT; (√19×√19); and, (1×1)HT. Above JcAs4 the (√19×√19) surface phase was quenched, such that the (1×1)LT and (1×1)HT structures merged to form a single (1×1) phase. The transitions to and from each of these surface phases were found to be reversible, occurring at very specific substrate temperatures for a given incident As4 flux. The activation energies (eA) characterizing the reversible surface phase transitions were measured and compared with those on the GaAs(100) surface.

Surface processes before and during growth of GaAs (001)

GaAs(001) surfaces in both metalorganic vapour phase epitaxy (MOVPE) and molecular beam epitaxy (MBE) have been studied under As-stabilization as well as during growth by reflectance anisotropy spectroscopy (RAS). During MBE growth simultaneously reflection high-energy electron diffraction (RHEED) measurements were performed. With increasing growth rate, the surfaces transform from a c(4 × 4) to a (2 × 4) in MBE. In MOVPE, with trimethylgallium (TMGa) and AsH 3 a transition occurs from a «c(4 × 4)-like» behaviour to a surface best described as a mixture of dielectric contributions from Ga-rich «(1 × 6)-like» and the As-rich «c(4 × 4)-like» regions. Time resolved RAS measurements show oscillations with monolayer periodicity in both epitaxial systems, however, with opposite phase. In MOVPE additionally data with time and spectral resolution are obtained which give insight into the surface modifications during a monolayer growth cycle. A model using an effective medium approach is proposed which relates the RAS oscillations during MBE growth to morphological changes associated with the island formation in the layer-by-layer growth modus. The MOVPE oscillation behaviour is more complex due to the final decomposition steps of trimethylgallium at the surface. In addition to the island formation it has to be included in the model that the surface oscillates between a state «more c(4 × 4)-like As dimers» and a state «more (1 × 6)-like As and Ga dimers»

The Au/CdTe interface: an investigation of electrical barriers by ballistic electron emission microscopy

The authors describe the application of ballistic electron emission microscopy to measure electrical barriers with nm spatial resolution. They show that the interface between Au and chemically treated CdTe is non-uniform and that problems of reproducibility are associated with small patches at the interface where the barriers are low.

The molecular beam epitaxial growth of GaAs/GaAs(111)B: doping and growth temperature studies

A series of investigations are presented which address various aspects of the growth, by molecular beam epitaxy, of n‐type (Si doped) on‐axis GaAs/GaAs(111)B. In situ characterization by reflection high‐energy electron diffraction has identified four surface phases on the static (zero growth rate) surface, and three reconstructions which occur, depending upon the substrate temperature, during growth. The n‐type doping properties of GaAs/GaAs(111)B epilayers have been compared with n‐GaAs/GaAs(100) structures. Hall effect and low‐temperature photoluminescence measurements have demonstrated that it is possible to dope GaAs/GaAs(111)B with Si in the 6×1014 to 1018 cm−3 range. A variable growth temperature study is also presented which examines the surface structural, electrical, optical, and surface morphological properties of n‐GaAs/GaAs(111)B grown in the 400 to 650 °C temperature range. The onset of electrical conduction, and optically active material, was found to be directly related to changes in the dynamic surface structure. The variable growth temperature study also revealed a temperature regime within which it was possible to significantly improve the surface morphology of on‐axis GaAs/GaAs(111)B structures whilst retaining good electrical and optical properties.

Effective barrier height, conduction‐band offset, and the influence of p‐type δ doping at heterojunction interfaces: The case of the InAs/GaAs interface

We demonstrate that the effective band discontinuity at an n‐isotype heterojunction interface can be significantly modified by introducing p‐type δ doping close to the interface during molecular beam epitaxy growth. This is shown for the case of the relaxed InAs‐GaAs interface where the band discontinuities with and without δ doping have been measured by the I‐V technique coupled with appropriate numerical modeling of the interface.

CH4/H2: A universal reactive ion etch for II‐VI semiconductors?

A high resolution reactive ion etching process, capable of producing nanostructures less than 50 nm wide in a variety of II‐VI semiconductors, is described. Using a mixture of methane and hydrogen, binary II‐VI compound, e.g., ZnTe, ZnSe, CdTe, ZnS, CdS, and ternary compounds, e.g., CdMnTe and ZnSSe, have been etched. It would appear that the CH4/H2 gas mixture will play the same role for the II‐VI semiconductors as it does for the III‐Vs, that of seemingly ubiquitous etching system.

Observation of photoluminescence from InAs surface quantum wells grown on InP(100) by molecular beam epitaxy

Photoluminescence (PL) measurements are presented for thin epitaxial layers of InAs, 2.5 A<d <36 A, grown on InP(100) by molecular beam epitaxy. The combination of efficient carrier capture and PL redshift with increasing InAs thickness clearly indicate the formation of InAs quantum wells on the InP surface. Data are also presented for InAs/InP structures capped with strained layers of either GaAs or In0.5 Al0.5 As. Since radiative recombination within the InAs layers can be distinguished from PL arising from both bulk and surface defects, this system allows us to monitor the quality of both the InAs/InP and InAs/air interfaces via their influence on the InAs quantum well luminescence.

A photoemission study of passivated silicon surfaces produced by etching in solutions of HF

Hydrogen-terminated silicon surfaces produced by etching in solutions of HF have been studied using photoelectron spectroscopy and LEED. They have been found to be remarkably free from contamination, locally ordered and electrically passivated. The hydrogen may be removed from the (100) surface by annealing in UHV at (520+or-10) degrees C. At that temperature, the initial (1*1) structure of the surface changes to a two domain (2*1) reconstruction and the Fermi level becomes pinned by surface states. The relationship between residual contamination and etching parameters such as time, concentration and crystallographic orientation of the sample has been examined, together with the degradation of the hydride surface with exposure to various environments. This has shown that the contamination levels of the surface are sensitive to etching procedure, but with suitable care, a 5% non-aqueous HF solution provides the optimum etch.

The molecular beam epitaxial growth of InAs on GaAs(111)B- and (100)-oriented substrates: a comparative growth study

Molecular beam epitaxy has been used to simultaneously grow epitaxial InAs onto (111)B- and (100)-oriented GaAs substrates, under a variety of growth conditions. Despite the large lattice mismatch InAs was observed to grow upon GaAs(111)B in a two-dimensional (layer-by-layer) manner, from onset of nucleation to micrometre thicknesses as indicated by clear, well streaked 2*2 reconstructed RHEED patterns throughout the deposition and smooth post-growth scanning tunnelling micrographs. The comparative study of InAs grown upon GaAs(100) followed the expected Stranski-Krastanow growth mode. Systematic variations of the epilayer thickness, substrate temperature and concentration of intentional n-type (Si) dopant, were carried out and the samples measured ex situ by the Hall effect and double-crystal X-ray diffraction. Despite the two-dimensional growth mode, the InAs/GaAs(111)B system was found to be electrically and structurally inferior to the InAs/GaAs(100) system for growth temperatures below 450 degrees C. However, above this temperature the systems became comparable in terms of both the measured electron mobilities and epilayer Bragg peak widths.