H. Qiang

Two‐dimensional electron gas effects in the electromodulation spectra of a pseudomorphic Ga0.78Al0.22As/In0.21Ga0.79As/GaAs modulation‐doped quantum well structure

We have studied the electroreflectance and photoreflectance spectra from a pseudomorphic Ga0.78Al0.22As/In0.21Ga0.79As/GaAs modulation‐doped quantum well (MDQW) structure in the temperature range 79<T<304 K. The features from the InGaAs MDQW can be accounted for on the basis of a two‐dimensional density of states and a Fermi level filling factor. A detailed line shape fit makes it possible to evaluate the Fermi energy, and hence the two‐dimensional electron gas concentration (Ns), as well as other important parameters of the structure. Our value for Ns is in good agreement with a Hall measurement.

Size dependence of the thermal broadening of the exciton linewidth in GaAs/Ga0.7Al0.3As single quantum wells

We have studied the temperature dependence of the linewidth, Γ(T), of the fundamental absorption edge in bulk GaAs and four GaAs/Ga0.7Al0.3As single quantum wells of different well width using photoreflectance. As a result of the size dependence of the exciton‐longitudinal optical phonon interaction, the thermal broadening of the linewidth diminishes as the dimensionality and size of the system are reduced.

Piezo-photoreflectance of the direct gaps of GaAs and Ga0.78Al0.22As

Abstract We have investigated the effects of compressive uniaxial stress (T) on the photoreflectance spectra at 300K of the fundamental direct band gap (E o ) and its spin-orbit split component (E o + Δ o ) of GaAs and Ga 0.78 Al 0.22 As for T along [001] and [110]. From the stress-induced shifts and splittings we have deduced the hydrostatic ( a ) and shear ( b and d ) deformation potentials of these materials.

Room-temperature photoreflectance characterization of pseudomorphic GaAlAs/InGaAs/GaAs high electron mobility transistor structures including the two-dimensional electron gas density

Using contactless photoreflectance at 300 K the authors have characterized three pseudomorphic GaAlAs/InGaAs/GaAs high electron mobility transistor structures. The spectra from the InGaAs modulation-doped quantum well (MDQW) channel can be accounted for on the basis of a step-like two-dimensional density of states (screened exciton) and a Fermi level filling factor. A detailed lineshape fit makes it possible to evaluate the Fermi energy, and hence the two-dimensional electron gas concentration. Furthermore, other important parameters of the system such as built-in electric fields, In composition and well width of the InGaAs MDQW channel can be evaluated.

Electroreflectance study of a symmetrically coupled GaAs/Ga0.77Al0.23As double quantum well system

Electroreflectance spectroscopy has been used to study a GaAs/Ga0.77Al0.23As symmetrically coupled double quantum well system as a function of externally applied electric field. Both intra‐ and inter‐well exciton transitions were detected. The energy shifts of the coupled quantum confined levels are in good agreement with a theoretical calculation. Details of the lineshape fit yield important information about the modulation mechanism.

A NEW OFFSET TECHNIQUE FOR SUPPRESSION OF SPURIOUS SIGNALS IN PHOTOREFLECTANCE SPECTRA

A new technique to suppress the unwanted, spurious signals often encountered in photoreflectance spectra from semiconductors and semiconductor microstructures is described here. This approach utilizes a properly phase‐shifted signal from the reference source that is applied to the lock‐in amplifier’s differential input. This paper demonstrates that this new approach is superior to existing methods of background minimization and is simpler to implement. It is particularly effective when the spurious signal is much larger than the underlying desired trace.

Temperature Dependence of Quantized States in Strained-Layer In0.21Ga0.79As/GaAs Single Quantum Well

The piezoreflectance (PzR) and photoreflectance (PR) measurements of a strained-layer (001) In0.21Ga0.79As/GaAs single quantum well as a function of temperature in the range of 20 to 300 K have been carried out. A careful analysis of the PzR and PR spectra has led to the identification of various excitonic transitions, mnH(L), between the mth conduction band state to the nth heavy (light)-hole band state. The parameters that describe the temperature dependence of E mnH(L) are evaluated. The detailed study of the temperature variation of excitonic transition energies indicates that the main influence of temperature on quantized transitions is through the temperature dependence of the band gap of the constituent material in the well. The temperature dependence of the linewidth of the 11H exciton is evaluated and compared with that of the bulk material.

Contactless electroreflectance characterization of three InGaAs quantum wells placed in a GaAs/AlGaAs resonant cavity

Abstract Contactless electroreflectance at 300 K has been used to characterize an InGaAs/GaAs/AlGaAs vertical-cavity surface-emitting laser test structure, i.e. three InGaAs quantum wells (QWs) placed in a GaAs/AlGaAs resonant cavity. The three observed resonances from the InGaAs QWs have made it possible to evaluate the well width and In composition. The 1020 nm lasing spectrum correlates very well with the energy of the lowest lying QW feature. Also, the built-in electric field in the structure has been determined from the Franz-Keldysh oscillations in the GaAs signal. Furthermore, there is some evidence for Be interdiffusion into the QW region.

Piezoreflectance and photoreflectance study of GaAs/AlGaAs digital alloy compositional graded structures

We have studied the piezoreflectance (PzR) spectra at 300 and 80 K related to the intersubband transitions from two different (001) GaAs/AlGaAs structures, an asymmetric triangular quantum well and a rectangular quantum well, fabricated by molecular beam epitaxy using the digital alloy compositional grading (DACG) method. A comparison of the relative intensity of heavy‐ and light‐hole related features in the PzR spectra and those in the photoreflectance emphasizes the contribution of the strain dependence of the energies of the confined states which allows us to identify the features associated with the heavy‐ and light‐hole valence bands unambiguously. Comparison of the observed intersubband transitions with the envelope function calculations provides a self‐consistent verification that the DACG method generated the desired potential profiles. Furthermore, the temperature dependence of both the energy position and broadening parameter of the fundamental conduction to heavy‐hole (11H) and light‐hole (11L)...

Nondestructive, room temperature analysis/qualification of wafer-sized semiconductor device structures using contactless electromodulation spectroscopy

Abstract This paper reviews the use of the contactless, and hence nondestructive, electromodulation methods of photoreflectance and contactless electroreflectance for the room temperature characterization and qualification of semiconductor device structures. These include heterojunction bipolar transistors, pseudomorphic high electron mobility transistors, quantum well lasers, vertical cavity surface emitting lasers, multiple quantum well infrared detectors and solar cells.