Photoreflectance Intensity Research Articles

Modeling the time-dependence of the photoreflectance spectroscopy on ultra-shallow As+ ion implanted silicon

Using a high throughput Photoreflectance (PR) spectroscopy setup, a significant temporal behavior of the PR spectroscopy from a wide range of silicon (Si) wafers is observed, induced by optical heating effect. The PR intensity and the built-in electric field enhance on a time scale of several minutes. A diffusion process-based time-dependent model is developed and validated on shallow Arsenic (As+) ion implanted Si layers. A theoretical form of the temporal built-in electric field is determined and used to fit the experimental PR intensity. Modeling the PR temporal effect enables the real time monitoring of As+ diffusion and the measurement of the diffusivity as a function of low implant energy (E: 3 – 7 keV) from which the lattice temperature is derived experimentally. Furthermore, the As+ Shockley-Read-Hall (SRH) carrier lifetime at 300 K is found to be strongly dependent over the same experimental E range. Using the SRH theory, defects formed by As+ implant are studied and related trap density (Ns) are measured versus E assuming unchanged As+ capture cross section.

Initial nucleation stage in photo-CVD of GeH4 on SiO2 substrate monitored by real-time spectroscopic ellipsometry and photo reflectance: Accurate determination of incubation time

The initial stage of photoexcited chemical vapor deposition (CVD) of Ge layers on SiO2 substrate was studied through a combined analysis of real-time spectroscopic ellipsometry and photoreflectance (PR). Just after GeH4 gas was admitted into the growth chamber, there was a certain period during which both the ellipsometric (Ψ, Δ) angles and PR intensity remained unchanged from their initial values. This phenomenon prior to steady-state growth is known as incubation, and it is due to the very slow reactive sticking of GeHx species on the dangling-bond free SiO2 surfaces. The incubation time was accurately determined from the time derivatives of (Ψ, Δ) and the PR intensity. In most cases, the incubation times independently derived from (Ψ, Δ) and the PR intensity coincided, but for other cases, those derived from the PR intensity were slightly longer than those derived from (Ψ, Δ). The incubation time decreased as thermally activated decomposition of GeHx species became prevalent above 300 °C. An activation energy of 18 kJ/mol was obtained by assuming that the rate of creating nucleation seeds on SiO2 is proportional to the inverse of the incubation time. Such a low activation energy, much lower than that of thermal CVD, is due to partial decomposition of GeH4 by photoexcitation to create reactive GeHx fragments. At the end of incubation, the Ge layer wetting the SiO2 surface changed into self-assembled Ge dots when more Ge atoms were deposited. The dot density was maximum at the beginning of dot formation and gradually decreased as the dot size became larger. The activation energy of continuous Ge growth on Ge-covered SiO2 surface was derived from the time evolution of Δ to be 13 kJ/mol. The minimum PR intensity for three-dimensional (3D) dot formation was lower than that of two-dimensional (2D) layer growth, reflecting greater surface roughness. Contrasting behaviors between 2D and 3D growth were also observed in terms of the recovery level of the PR intensity after prolonged growth; continued 2D growth recovered up to 90% of the initial level, while 3D dot formation recovered only 50 −60%.

Optical features of InAs quantum dots-in-a-well structures

Electronic energy structure and features of optical interband transitions of InAs quantum dots-in-a-well structures are studied via photoreflectance (PR) and temperature-dependent photoluminescence (PL) spectroscopy. InAs dots were grown with and without InGaAs capping layer and embedded in GaAs/AlAs quantum wells. Experimental results revealed that the 5 nm thick InGaAs capping layer significantly improves PR and PL signal intensity. Moreover, a shift of the quantum dot ground-state optical transition to lower energy by about 120 meV was observed. The red-shift of the ground-state transition is associated mainly with an increase of dot size and decrease of strain within quantum dots. Furthermore, the origin of PL intensity quenching with temperature is discussed in terms of electronic energy structure revealed from PR spectra and calculations performed within effective mass approximation.

Optical features of InAs quantum dots-in-a-well structures

Electronic energy structure and features of optical interband transitions of InAs quantum dots-in-a-well structures are studied via photoreflectance (PR) and temperature-dependent photoluminescence (PL) spectroscopy. InAs dots were grown with and without InGaAs capping layer and embedded in GaAs/AlAs quantum wells. Experimental results revealed that the 5 nm thick InGaAs capping layer significantly improves PR and PL signal intensity. Moreover, a shift of the quantum dot ground-state optical transition to lower energy by about 120 meV was observed. The red-shift of the ground-state transition is associated mainly with an increase of dot size and decrease of strain within quantum dots. Furthermore, the origin of PL intensity quenching with temperature is discussed in terms of electronic energy structure revealed from PR spectra and calculations performed within effective mass approximation.

Note: A signal-to-noise ratio enhancement based on wafer light irradiation system for optical modulation spectroscopy measurement

We have recently found that the magnitude of the photoreflectance (PR) signal ΔR∕R on silicon wafers depends on the duration of continuous probe or pump beams irradiation. This temporal behavior of the ΔR∕R signal is attributed to the defects related electronic states at the Si∕ SiO(2) interface, which could be modified by the optical irradiation. Prior to the actual measurement, an optical irradiation of the silicon on insulator or ion implanted Si wafer can significantly enhance the signal-to-noise ratio of the PR intensity and, therefore, improve the goodness of fit. Such phenomena can be exclusively seen using a rapid detection system. A new design of the method is reported.

Photoreflectance spectroscopy of self-organized InAs/InP(0 0 1) quantum sticks emitting at 1.55 μm

Photoreflectance (PR) measurements are performed as a function of temperature on self-organized InAs/InP(0 0 1) quantum sticks (QSs) grown by solid-source molecular beam epitaxy. With a very weak excitation power, three PR transition energies are arising and associated with the ground state and two excited states, respectively, in good agreement with both photoluminescence (PL) and PL excitation measurements. The temperature dependence of the PR transition energies is in good agreement with the Bose–Einstein behavior. From PL analysis of these InAs/InP QSs, the ground state was assumed to be partially filled because of the residual n-type doping of the InP barrier layers. The PR spectra analysis allows us to further confirm this assumption, considering mainly the relative PR intensity of the different transitions, as well as the Franz Keldysh oscillations (FKO) above the InP bandgap.

Spectroscopic characterization of naturally and chemically oxidized silicon surfaces

We have determined the thicknesses of naturally and chemically grown oxides on HF-cleaned silicon surfaces in ambient air and in NH4OH/H2O2/H2O solution, respectively, using spectroscopic ellipsometry. The naturally grown oxide thickness versus air-exposure time plots yield a rate constant of 3.5±0.5 Å/decade in ambient air. Chemical oxidation occurs immediately upon immersing the sample in the chemical solution and leaves the sample surface terminated with ∼6 Å of a chemical oxide. Photoreflectance intensity is found to be strongly dependent on such surface processing, and results are explained by the different degree of surface (interface) states.

Schottky barrier height and interfacial state density on oxide-GaAs interface

Photoreflectance (PR) and Raman spectra were employed to investigate the interfacial characteristics of a series of oxide films on GaAs. The barrier heights across the interfaces and the densities of interfacial states are determined from the PR intensity as a function of the pump power density. The oxide-GaAs structures fabricated by in situ molecular beam epitaxy exhibit low interfacial state densities in the low 1011 cm−2 range. The density of the interface states of the Ga2O3(Gd2O3)–GaAs structure is as low as (1.24±0.14)×1010 cm−2. The Ga2O3(Gd2O3) dielectric film has effectively passivated the GaAs surface. Additionally, Raman spectra were used to characterize the structural properties of the oxide films.

Correlation between photoreflectance signal intensity and current gain of InP/InGaAs heterojunction bipolar transistor structures

InP/InGaAs heterojunction bipolar transistor (HBT) wafers grown by metal–organic vapor-phase epitaxy were characterized by photoreflectance (PR) spectroscopy. We found that the intensity of PR signals from the InP emitter and InGaAs collector layers of the HBT wafer decreases with increasing emitter growth temperature and shows a linear positive correlation with the HBT current gain. On the other hand, the intensity of PR signals from the n-InP single layers scarcely changes with increasing InP growth temperature. Similar tendencies in the PR intensity were also observed with changes in postgrowth annealing temperature. The change in the PR intensity of the emitter and collector layers is expected to reflect the crystal quality of the adjacent InGaAs:C base layer, which determines the HBT current gain. The present PR method is eminently suitable for the nondestructive diagnostics of the crystal quality of InP/InGaAs HBT wafers.

Characteristics of three-beam photoreflectance in ZnTe/GaAs with deep traps

The characteristics of surface built-in electric field in ZnTe/GaAs with deep traps was studied by using three-beam photoreflectance (PR). Ar-ion laser-modulated PR intensity was very weak. When Ar-ion laser was used as modulation source, PR intensity under He–Ne laser bias was stronger than that under no bias irradiation, whereas He–Ne laser was used as modulation source, PR intensity under Ar-ion laser bias was weaker than that of under no bias irradiation. These results could be explained by competitive process in direction of modulated electric field between deep trap levels and band-to-band. As the temperature is increased from 15 K to about 90 K above band gap excitation, PR intensity was increased, whereas below band gap excitation, PR intensity was decreased. The increase in temperature results in increasing of probability of thermal emission rate of deep trap levels.

Nondestructive Analysis of Current Gain of InP/InGaAs Heterojunction Bipolar Transistor Structures using Photoreflectance Spectroscopy

AbstractInP/InGaAs heterojunction bipolar transistor (HBT) wafers grown by metal-organic vapor phase epitaxy (MOVPE) have been characterized by photoreflectance (PR) spectroscopy. The intensity of PR signals from the InP emitter and InGaAs collector layers of the HBT wafers decreased with increasing emitter growth temperature and showed a linear positive correlation with the HBT current gain. On the other hand, the intensity of PR signals from the n-InP single layers scarcely changed with increasing InP growth temperature. The change in the PR intensity of the emitter and collector layers is expected to reflect the crystal quality of the adjacent InGaAs:C base layer, which determines the HBT current gain. Thus, the present PR method is eminently suitable for the nondestructive analysis of the current gain of InP/InGaAs HBT wafers.

Photoreflectance characterization of CdTe thin films grown by Molecular Beam Epitaxy on GaAs(100)

CdTe films have been grown on top of GaAs(100) by means of Molecular Beam Epitaxy (MBE) at 300 °C substrate temperature. Different procedures for the CdTe growth and for the preparation of the GaAs substrates resulted in diverse crystalline qualities of the CdTe films. We present the results obtained from PhotoReflectance (PR) measurements of these films employing HeNe and Ar-ion lasers as modulating excitation. For Ar excitation, the ratio of CdTe to GaAs signal strength for the E0 transition is enhanced, allowing a differentiation of the contributions from film and substrate. Both the PR line shape and intensity are correlated to the structural quality of the CdTe films. One of the samples presented a below-band-gap transition of the GaAs substrate around 30±5 meV from E0 which is attributed to donor states produced by Te atoms diffused in the interface; this result demonstrates the high sensitivity of the photoreflectance technique to the structural properties of interfaces.

Spectroscopic studies of strained-layer GaSbAlSb superlattices

Photoreflectance (PR) and photoluminescence (PL) studies of GaSbAlSb multiple quantum well structures under hydrostatic pressure at cryogenic temperatures are presented. A large number of excitonic transitions from the conduction sub-hands (CB) to the light (LH) and heavy (HH) hole sub-bands are observed in PR. In contrast, the ground sub-band transition from the CB to the LH dominates the PL. The pressure coefficients get progressively smaller for the higher sub-band transitions. The PL intensity drops dramatically as the direct Γ CB crosses the L CB at ∼ 10 kbar but the PR intensity does not. However a large drop in PR signal is observed as the X CB crosses each sub-band energy.

High Pressure Optical Studies of GaSb-AlSb Multiple Quantum Wells

AbstractThe pressure coefficients (α) of the excitonic transitions arising from the conduction (CB) to the heavy (HH) and light (LH) hole sub-bands of a GaSb-AlSb multiple quantum well structure (MQW) grown on a GaAs substrate are determined. Photoreflectance (PR) and Photoluminescence (PL) studies are employed at cryogenic temperatures. The α for the ground sub-band transition is ~10% smaller than that for bulk GaSb. This effect is explained by in-plane deformation of the MQW arising from the substrate under external pressure. On the other hand, the α get progressively smaller for the higher sub-band transitions due to quantum confinement The PL intensity drops dramatically as the direct Γ CB crosses the L CB at ~10kBars but the PR intensity persists until the X CB crosses each sub-band energy.

Excitation wavelength and pump chopping frequency dependence of photoreflectance in Hg1-xCdxTe

We have measured photoreflectance (PR) at 77 K in the vicinity of the E1 optical feature from Hg1-xCdxTe (0.14 < x < 0.3) using infrared (3.39 μm) and deep blue-violet (4579 and 4067 Å) pump beams; both n- and p-type materials were investigated. The spectral lineshapes were fitted using existing theories of electromodulation yielding interband energies and broadening parameters. In addition, the dependence of the PR intensity on the modulation frequency (f) was studied in order to obtain information regarding surface (interface) traps. The variation of PR intensity with f yielded characteristic trap time constants in the range 1–4 ms.