Reflection Mass Spectrometry Research Articles

Ultrahigh Vacuum Atomic Layer Epitaxy of Ternary II-VI Semiconductor Compounds

A concise discussion concerning the UHV ALE growth of ternary II-VI compounds is presented in this paper. Simultaneous reflection mass spectrometry (REMS) and reflection high energy electron diffraction (RHEED) measurements of the surface kinetic and structural parameters, respectively, governing the UHV ALE growth of Cd 1-x Zn x Te and Cd 1-x Mn x Te heteroepitaxial films are reported. In addition, a Monte-Carlo-based method for simulation of the UHV ALE process of CdTe (the model-compound for this growth technique) has been used for investigation of the Cd cation's fluxes reflected from the growing epilayer surface in different phases of the ALE process. The Cd + ion-related REMS signals measured during CdTe growth have been compared with the simulation results.

Ultrahigh vacuum atomic layer epitaxy of CD 1 − xMn xTe layers grown on [formula omitted] (100) substrates: reflection mass spectrometry studies

Simultaneous reflection mass spectrometry (REMS) and reflection high energy electron diffraction (RHEED) measurements of the surface kinetic and structural parameters, respectively, governing the UHV ALE growth of Cd 1 − x Mn x Te films are reported. A set of six experiments have been performed. Three of them concerned Cd 1 − x Mn x Te UHV ALE, and one Cd 1 − x Mn x Te MBE growth. For comparison, one experiment was also performed concerning UHV ALE of Cd 1− x Zn x Te and one concerning UHV ALE of CdTe, with two sequent cation deposition pulses. The REMS signals for Cd + cations during the deposition and ‘dead’ times of the ALE cycle were measured. RHEED patterns were also recorded to control the surface reconstruction of the epilayer during different deposition stages of the ALE cycle. An interesting phenomenon was observed in the REMS Cd + signal behaviour during the ALE growth of Cd 1 − x Mn x Te and Cd 1 − x Zn x Te. It consists in an evident increase of the intensity of this signal occurring in cases when both cation elements (Cd + Mn or Cd + Zn) are impinging onto the substrate surface simultaneously. These changes in Cd + REMS signals (15% and 5% in the cases of Cd + Mn and Cd + Zn simultaneous deposition, respectively) have been attributed to effects of mechanical impacts between ‘hot’ Mn and a bit ‘cooler’ Zn atoms with weakly bound ‘cool’ Cd atoms which are thermally accommodated to the substrate surface. The same mechanism, namely mechanical impact of ‘hot’ Mn atoms with ‘cool’ Cd atoms is believed to be responsible for the measured differences in chemical composition of the ternary compound Cd 1 − x Mn x Te, when grown by MBE ( x = 0.4) or UHV ALE ( x = 0.06), at the same growth parameters applied.

Reflection mass spectrometry studies on UHV ALE of Cd 1− xZn xTe (0≤ x≤1) compounds

Simultaneous reflection mass spectrometry (REMS) and reflection high energy electron diffraction (RHEED) measurements of the surface kinetic and structural parameters, respectively, governing the UHV ALE growth of ZnTe and CdZnTe films are reported. Two sets of experiments have been performed. In the first one we have deposited several ZnTe ALE layers with different Zn/Te flux ratios, timing cycles and growth temperatures. The REMS signals for Zn + cations during the deposition and `dead' times of the ALE cycle were measured; RHEED patterns were also recorded to control the smoothness of the epilayer. In the second part of our experiments a set of CdZnTe layers was grown by ALE. During the growth, the REMS signals were measured for both Zn + and Cd + cations. An interesting phenomenon is observed in the REMS Zn + signal behavior during the ALE growth of ZnTe. In certain growth conditions, the REMS Zn + signal increases in the presence of the Te 2 molecules on the surface of the growing layer, probably due to partial reevaporation of the first chemisorbed layer of Zn. Similar behavior of the REMS Zn + signal is observed for ALE growth of Cd 0.5Zn 0.5Te layer. However, in this case no changes in the Cd + signal can be seen when the Te cell is opened.

Real-time thickness control of resonant-tunneling diode growth based on reflection mass spectrometry

We used reflection mass spectrometry (REMS) to control the molecular beam epitaxial growth of resonant-tunneling diode (RTD) structures. The REMS-based thickness value controlled the effusion cell shutter actuation, and compensated in real time for flux transients and short-term flux drift. Use of REMS control during deposition of AlAs, InAs, and InGaAs layers resulted in improved symmetry and reproducibility of the RTD I-V characteristics, compared with time-based, dead-reckoning growth. REMS-based control for flux compensation during growth of GaAs/AlGaAs multi-quantum-well structures is also reported.

Detection and reduction of indium segregation during molecular-beam epitaxial growth of InGaAs/GaAs using in situ reflection mass spectrometry

The use of reflection mass spectrometry (REMS) as an in situ diagnostic for surface segregation of indium during InGaAs layer growth is reported. InGaAs growth at substrate temperatures above 500 °C yields a REMS signature which indicates a thickness-dependent surface In content. Adapting a simple segregation model [K. Muraki, S. Fukatsu, Y. Shiraki, and R. Ito, Appl. Phys. Lett. 61, 557 (1992)], the segregation ratio R was extracted under various deposition conditions in real time. The segregation ratio obtained during InGaAs/GaAs multiple quantum well growth at 500–530 °C suggests the presence of InGaAs composition grading near interfaces, and agrees qualitatively with ex situ characterization by x-ray diffraction. The high values of R (0.7–0.8) observed under normal device-layer growth produces a surface layer with high In content (i.e., InAs). The segregation ratio was not sensitive to unrelaxed layer strain, but showed a large increase under conditions which probably produce island (three-dimensional) growth. Methods for preparation of high-quality InGaAs layers with more abrupt AlGaAs/InGaAs/GaAs heterointerfaces are discussed, such as use of a two-step growth procedure or the incorporation of thin ‘‘cap layers’’ prior to high temperature AlGaAs growth.

Molecular-beam epitaxy flux transient monitoring and correction using in situ reflection mass spectrometry

The use of reflection mass spectrometry (REMS) for in situ monitoring of group III flux transients during molecular-beam epitaxial growth is reported. Normal shutter closure caused effusion cell heating which resulted in flux overshoots of up to 30%. The variation of the magnitude and time constant of the overshoot, as a function of group III material and shutter closed time, was determined for our Riber 2300 system. It was verified that the observed REMS transients corresponds to growth transients using photoluminescence and transmission electron microscopy analysis of GaAs/AlGaAs and InGaAs/InAlAs multiple quantum well (MQW) structures. Flux transient correction during GaAs/AlGaAs MQW growth was accomplished by ramping the temperature of the Ga effusion cell, based on our previous analysis of the transients.

Application of reflection mass spectrometry to molecular-beam epitaxial growth of InAlAs and InGaAs

Reflection mass spectrometry (REMS) has been used to monitor In fluxes leaving the surface, and to optimize growth conditions during molecular-beam epitaxial (MBE) growth of InAlAs and InGaAs on InP. The optical, electrical, and structural properties of the films are shown to be a function of the measured indium signal. This optimization technique is independent of thermocouple or pyrometer readings to set the growth temperature, and has led to significant improvements in the optical and electrical properties of lattice matched InAlAs on InP. REMS-optimized unintentionally doped InAlAs has been grown with electron concentrations as low as 3×1015/cm3, electron mobilities as high as 4 000 cm2/V s at 77 K, and photoluminescence linewidths as narrow as 10 meV. The high quality of these layers is attributed to reduced alloy clustering and reduced ionized impurity scattering.